Other ideas:
I know that DriveThruRPG had the Novus beta rules up for free for a long, long time. It's a class and level-based RPG that uses a 2d10 mechanic instead of d20. I really liked the beta rules, though I haven't played the game.
Fantasy Craft is an amazing, if very crunchy, interpretation of the d20 system (but not free).
If you want a "steampunk"/"electrotech" d20 variant, definitely check out Radiance RPG, which has a free core rulebook PDF available right on the creator's web site.
The D6 system seems like it might be a good bet as well, with fantasy, "modern"/pulp, and sci-fi flavors already out there.
I personally dislike both of the more "realistic" 3d6-roll-under systems in GURPS and HERO, but they have a lot of adherents out there. Most people who find that it's their "thing" generally will put both of those at the top of their RPG list.