Cut N Paste job from Spycraft Wiki, sorry - don't have time to reconnect all the links - but google is your friend...
Personally I think a lot of 40K figures fit the bill especially Imperial types (or Tau depending on your aliens, or just using helmeted ones), and there's usually plenty of 2nd hand ones around.
Modern
Hasslefree 28mm UK Hasslefree reviews (1 Review) - Kev White's miniatures company - the 'Adventurers' range is the one you want for modern figures, with a variety of 'zombie hunters' as well as a few figures clearly inspired by films.
SJ Games 28mm US SJ reviews Mostly out of print, though according that site Steampunk Miniatures are still available. They also do some Cardboard Heroes - sheets of heroes that can be punched out and folded up and used as miniatures.
Ground Zero Games 25mm UK GZG reviews - Looks to be quite a selection - though be warned most don't have pictures. The Street Level range seems to be where most generic modern figures lie, though the 'Characters' section has some and if you dare to look there are some Bond-Girl-esque types in the 'Gentlemans Collectables' section - though you have to scroll through some gratuitous pewter nudity to reach them.
Tactical Miniatures 28mm US Tactical reviews - Small selection consisting of various types of police, terrorists and hostages. The hostages tempt me as they are the one type of civilian that you want to know where they are in the room and I've not seen anyone else do them.
Superfigs 28-30mm US Superfigs reviews - Mostly superheroes, but some civilians packs in and amongst.
Copplestone 28mm UK Copplestone reviews - The 'Future Wars' section is what you want - has a good selection of cops, men in black, bikers and the like.
Wargames Foundry 28mm UK Foundry Reviews (1 Review) - The Foundry's Street Violence line is a an excellent collection of all sorts of modern heroes and antagonists, including soldiers, cops, gangsters, rioters, killer street walkers, bikini babes, angry midgets, and more!
The Assault Group 28mm US TAG Reviews - Large line of military and swat miniatures, including terrorists and hostage miniatures. Also sells 28mm scale weapons for equiping miniatures with accurate weaponry. It should be noted that their site is notoriously slow and buggy. It is reccomended that you shop through a retailer (The Hobby Box for example).
Military
Black Scorpion 32mm UK Black Scorpion reviews (1 Review)Adam Clarke's miniatures company - Does some Iraqi Militia & U.S. Marines, a couple of the Desolation Row figures are more 'adventurer' style modern figures too.
Steve Johansen Miniatures (can't find a scale) US Steve Johansen reviews - Seems to be mostly military on the site with U.S., Terrorists, Russians & South African forces.
Copplestone 28mm UK Copplestone reviews - The 'Future Wars' section is what you want - the troopers are generic, though look US inspired to me.
Sci-Fi
Hasslefree 28mm UK Hasslefree reviews (1 review) Kev White's miniatures company - A few ranges of Sci-Fi though some rank as slightly bizarre. A bit WH40K inspired in that his major range is the Kjaran's (Space Dwarves). A lot of the Pirates range, suit slightly odd sci-fi as well.
Heresy 28mm UK Heresy reviews Andy Foster's miniatures company - not many sci-fi figures but has a small range mostly with customisable weapons. and could fit for some modern games as well. As a note some of his mosters/demons could make decent aliens as well.
Em4 28mm UK Em4 reviews Notable for their plastics which are a decent and cheap way to round out slightly futuristic cops/military. There is also large range of metals with a futuristic theme.
Ground Zero Games 25mm UK GZG reviews - The Star Grunt range is sci-fi though on a cursory search through I could find no pictures, which always makes me wary