I put this list together for over on the D20 modern board at wizards.com, and I figured folks here would appreciate it too. I'll be keeping the list updated over there, however.
Modern Miniatures
General
Street Violence - http://www.wargamesfoundry.com/collections/SV/index.asp
Future Wars - http://www.gisby.org/fw.htm
Future Shock - http://www.princeaugust.ie
Modern Heroes - http://www.eclipseminiatures.com/
em4's Combat Zone & Scifi lines - http://www.crystalcaste.com/ or http://www.em4miniatures.com/acatalog/shop.html
Chinatown - http://www.stonecirclegames.com/china-info.htm or http://www.newwave.org/
Shadowforge scifi, character & manga lines - http://www.shadowforge.com.au/
Eureka Miniatures (Lotta stuff from Victoriana to WW1 to nam, to modern police & scifi) - http://www.eurekamin.com.au/
Ground Zero Games (lotta stuff suitable in lotta ranges) - http://www.gtns.net/gzgstore/
Iron Wind Metals (shadowrun) - http://www.ironwindmetals.com/
Excalibur Miniaturen (scifi minis from germany) - http://www.excalibur-miniaturen.com..._e/exca_eng.htm
i-Kore (Void l line) - http://www.i-kore.com/
The London War Room (A little of everything; military figs, victorian, figs, buildings...) - http://www.thelondonwarroom.com/
Chiltern Miniatures (mil figs, ww2 to present, scifi figs, some SE asian buildings) - http://www.chilternminiatures.co.uk/index.html
North Star Figures (mostly military figs, but also 1/48 scale mil models (some motorized, run down the old minis!) ww1, gangs of NYC figs, and 28mm Dinos!) - http://www.northstarfigures.com/
Military
Devil Dog Designs (modern military & terrorists) - http://www.devildogdesign.com/home.htm
West Wind Productions (WW2 & Nam minis) - http://www.westwindproductions.co.uk/index.asp
The Assault Group (More modern mil) - http://www.theassaultgroup.com/main.html
Artizan Design (WW2 era) - http://www.northstarfigures.com/shop/artizandesign/
Old Glory Miniatures (Vietnam era ($1/fig, way cheap, way nice!), 25mm military vehicles, including Huey and Cobra choppers!, 25mm buildings for various regions and eras) - http://www.oldgloryminiatures.com/Vietnam.asp
Brittania Miniatures (20mm mil vehicles, 25mm modern minis) - http://www.britanniaminis.com/
Baker Company (Nam minis) - http://www.bakercompany.co.uk/
Terrain & Vehicles
Ainsty (terrain) - http://www.ainsty.co.uk/index.htm
Armorcast (terrain & vehicles) - http://www.armorcast.com/
SHQ (20mm military vehicles, undersized but better than nothing) - http://www.shqminiatures.co.uk/Sitefiles/main.php
DLD Production (military vehicles) - http://www.dldproductions.com/
Ultra-Cast (terrain & military vehicles) - http://www.ultra-cast.net/
Minute Men Toy Soldiers (WW2 military vehicles, 20mm scale) - http://www.mmtoysoldiers.com/modelmain.htm
Scale Model Accessories LTD (ww2 vehicles, 1/76 scale) - http://www.smal.sageweb.co.uk/
War Factory (DIY instructions for easy scenery and accessories) - http://www.warfactory.co.uk/
Scenic Effects (with a name like that, what do you expect?) - http://www.geocities.com/sceniceffects/
Paper
from various publishers - 21st Century Heros, Main Street USA, Twilight Street, Bargain Wheels, Panzerkämpfrüstung 62 Ausf G Battlesuit , Papertown, Paper Violence - all paper minis of varying degrees of complexity - http://www.rpgnow.com
Sparks paper mini fonts - http://www222.pair.com/sjohn/sparks.htm
and if you have the wherewithall to actually work your way through all 1,296 manufacturer entries, you can rummage on The Miniatures Page for more manufacturers of suitable minis. - http://theminiaturespage.com/man/
Looking for more vehicles? Diecast replicas and plastic or resin model kits in 1/60(30mm) to 1/76(20mm) scale will look approximately right with 25-28mm minis, depending upon the type of vehicle (a 1/76 scale tank looks more "right" than a 1/76 scale motorcycle or car, for example)
Long Gone Miniatures for your ebaying pleasure
Grendel - Made resin vehicles for at least one scifi game (kryomek), including a recon car that looks very much like the spinner from blade runner.
RAFM - the old large scale Heavy Gear figures make excellent man-sized suits of power armor. They also manufactured some minis for Cyberpunk, a personality (non-mecha) minis for Mekton, both of which were suitable for a modern genre game
Modern Miniatures
General
Street Violence - http://www.wargamesfoundry.com/collections/SV/index.asp
Future Wars - http://www.gisby.org/fw.htm
Future Shock - http://www.princeaugust.ie
Modern Heroes - http://www.eclipseminiatures.com/
em4's Combat Zone & Scifi lines - http://www.crystalcaste.com/ or http://www.em4miniatures.com/acatalog/shop.html
Chinatown - http://www.stonecirclegames.com/china-info.htm or http://www.newwave.org/
Shadowforge scifi, character & manga lines - http://www.shadowforge.com.au/
Eureka Miniatures (Lotta stuff from Victoriana to WW1 to nam, to modern police & scifi) - http://www.eurekamin.com.au/
Ground Zero Games (lotta stuff suitable in lotta ranges) - http://www.gtns.net/gzgstore/
Iron Wind Metals (shadowrun) - http://www.ironwindmetals.com/
Excalibur Miniaturen (scifi minis from germany) - http://www.excalibur-miniaturen.com..._e/exca_eng.htm
i-Kore (Void l line) - http://www.i-kore.com/
The London War Room (A little of everything; military figs, victorian, figs, buildings...) - http://www.thelondonwarroom.com/
Chiltern Miniatures (mil figs, ww2 to present, scifi figs, some SE asian buildings) - http://www.chilternminiatures.co.uk/index.html
North Star Figures (mostly military figs, but also 1/48 scale mil models (some motorized, run down the old minis!) ww1, gangs of NYC figs, and 28mm Dinos!) - http://www.northstarfigures.com/
Military
Devil Dog Designs (modern military & terrorists) - http://www.devildogdesign.com/home.htm
West Wind Productions (WW2 & Nam minis) - http://www.westwindproductions.co.uk/index.asp
The Assault Group (More modern mil) - http://www.theassaultgroup.com/main.html
Artizan Design (WW2 era) - http://www.northstarfigures.com/shop/artizandesign/
Old Glory Miniatures (Vietnam era ($1/fig, way cheap, way nice!), 25mm military vehicles, including Huey and Cobra choppers!, 25mm buildings for various regions and eras) - http://www.oldgloryminiatures.com/Vietnam.asp
Brittania Miniatures (20mm mil vehicles, 25mm modern minis) - http://www.britanniaminis.com/
Baker Company (Nam minis) - http://www.bakercompany.co.uk/
Terrain & Vehicles
Ainsty (terrain) - http://www.ainsty.co.uk/index.htm
Armorcast (terrain & vehicles) - http://www.armorcast.com/
SHQ (20mm military vehicles, undersized but better than nothing) - http://www.shqminiatures.co.uk/Sitefiles/main.php
DLD Production (military vehicles) - http://www.dldproductions.com/
Ultra-Cast (terrain & military vehicles) - http://www.ultra-cast.net/
Minute Men Toy Soldiers (WW2 military vehicles, 20mm scale) - http://www.mmtoysoldiers.com/modelmain.htm
Scale Model Accessories LTD (ww2 vehicles, 1/76 scale) - http://www.smal.sageweb.co.uk/
War Factory (DIY instructions for easy scenery and accessories) - http://www.warfactory.co.uk/
Scenic Effects (with a name like that, what do you expect?) - http://www.geocities.com/sceniceffects/
Paper
from various publishers - 21st Century Heros, Main Street USA, Twilight Street, Bargain Wheels, Panzerkämpfrüstung 62 Ausf G Battlesuit , Papertown, Paper Violence - all paper minis of varying degrees of complexity - http://www.rpgnow.com
Sparks paper mini fonts - http://www222.pair.com/sjohn/sparks.htm
and if you have the wherewithall to actually work your way through all 1,296 manufacturer entries, you can rummage on The Miniatures Page for more manufacturers of suitable minis. - http://theminiaturespage.com/man/
Looking for more vehicles? Diecast replicas and plastic or resin model kits in 1/60(30mm) to 1/76(20mm) scale will look approximately right with 25-28mm minis, depending upon the type of vehicle (a 1/76 scale tank looks more "right" than a 1/76 scale motorcycle or car, for example)
Long Gone Miniatures for your ebaying pleasure
Grendel - Made resin vehicles for at least one scifi game (kryomek), including a recon car that looks very much like the spinner from blade runner.
RAFM - the old large scale Heavy Gear figures make excellent man-sized suits of power armor. They also manufactured some minis for Cyberpunk, a personality (non-mecha) minis for Mekton, both of which were suitable for a modern genre game