You can download a free version of
Star Frontiers (an old space/sci-fi game by TSR) at starfrontiersman.com
Havn't read through all of it myself but it seems interesting.
There's also a number of 'currently' supported games that have 'quickstart' rules that you can download and get a fair bit of playability out of them.
Castles and Crusades (basically AD&D with a Unified system called SIEGE) It only features the classic 4 classes.
http://www.trolllord.com/cnc/ccqs.html I actually found that there is a fair bit of errors/inconsistencies in these rules, but with a discerning eye quite playable.
Lightspeed (a FUSION based sci-fi game. The quickstart rules try to corner you into playing space-rangers, there are a couple other options you could also download to expand that.) Lightspeed is menat to cover all the tropes of space games and I think it does it quite well.
Lightspeed
HARP (High Adventure RolePlaying) also has some kind of quickstart rules. I really havn't looked at them though.
HARP check the 'free downloads' menu and look for HarpLite
Speaking of Lite
MicroLite d20 is pretty awesome if all you want to do is play a really simple version of D&D 3e ish (that eliminates Vancian magic by the way) recommend downloading the Microlite d20 Purest Essence
www.microlited20.net (just scroll down)