I can't reccomend d20 Modern highly enough. IMO, it's an improvement on 3.0 D&D in essentially every way, and in many ways superior to 3.5 as well.
First, d20 Modern dispenses with the Your Class Is Who You Are mode of D&D, which is a massive benefit in my book. The next time I hear a player ask me why my "thief" can't pick locks and doesn't want to steal from the party will be too soon; in d20 Modern, you are who you want to be, unfettered by class archtypes that were cool two decades ago but look a little long in the tooth after the hundreth session of D&D.
Second, d20 Modern cuts down to size the massively cumbersome magic system of standard d20. It doesn't require magic at all (and certainly has no 'wealth by level guidelines' or golfbags of magic weapons), and if you do introduce magic, you can do so at a much reduced and more playable scale. Personally, I like to use Arcana Unearthed's more flavorful (and versatile) spells with the level caps and entry requirements of d20 Modern; your mileage may vary.
Third, d20 Modern dispenses with the wealth-by-level guidelines, and, indeed, with the items-as-power mentality of core D&D. Level, skills and feats matter; equipment does, too, but you're more likely to procure it at need rather than amass great mounds of treasure. I already mentioned this, but it bears repeating.
Fourth, d20 Modern is a (cut-down) all-in-one book. One stop shopping for all your GMing needs, provided you're a skilled homebrewer. True, you can get the whole SRD printed out for free, but a book is much more convenient.
Fifth, d20 Modern makes a nice compromise between 3.0 and 3.5 when it comes to critical hits. Crit enhancers stack (as in 3.0), but they only add 1 regardless of the initial traits of the weapon. This keeps crit-stacking under control but seems more fun and flavorful than 3.5's gutting of it.
Now, d20 Modern vs. Grim Tales... I'm one of the very few on ENWorld to prefer the former to the latter. Both are much superior to D&D 3.x, IMO, but I like d20 Modern's system better. I like advanced and prestige classes, which d20 Modern retains and Grim Tales dumps. Mongoose has released some interesting d20 Modern-ish games in its OGL line: their Steampunk volume is particularly nice. Spycraft is a system I haven't had the chance to play, but it does look very elegant at a glance, and perhaps worth a look. I'd get only one of these; there's just too much overlap between them.
If you want more fantasy and less modern, I'd also suggest Mongoose's Conan the RPG; there's a sword and sorcery system different enough to get players' blood pumping! Very good product. Even if you just want to play a more standard fantasy game, its magic system, classes and suggested GMing style are all a step up from D&D, and it, like d20 Modern or Grim Tales, is a single, self-contained volume. Unlike either d20 Modern or Grim Tales, it has received extensive support from its company of origin, should you want further material.
