Lately I've been cruising my old d20 RPG haunts. One of my favorite is RPGObjects (http://www.rpgobjects.com/). These guys have written some great tabletop gaming material. One of my favorites is the Blood & Space line. Chuck Rice authors most of the game material and does a great job, IMO. Recently RPGO has branched out in True 20 conversions as well as creating their own d20 mechanics variant called "Modern20".
Just so you know... d20 rules are based on the Wizards of the Coast's d20 opensource gaming license. The notion here is that a multitude of 3rd party publishers could reference or even quote the WotC rules as part of the publisher's products. One game system to unite them all and in the darkness bind them! D20 is generally pretty flexible and well known. Unfortunately, WotC has a tendency to need to publish new editions which invalidate much of the older rules. A pain to keep up with. Then there are the d20 flaws which are inherit in the system and what I would call "legacy" to the Advanced Dungeons & Dragons rules. But from this generic d20 system have emerged a number of very good deviations.
I've mentioned True 20 before. I'll follow-up by simply saying True 20 is an easier to learn/use version of WotC's d20 system.
The other major variant I know of is Spycraft by Crafty Games. Spycraft's system is like d20-zilla. It is a hugely expanded and much more complex version of d20. The advantage here is that Spycraft pretty much has a rule for everything. This can be really useful for certain things, but obviously there is a fine line with playability.
I have not researched Modern20 from RPGO enough to really qualify it, but I suspect is will be more rule heavy than d20, but probably not as heavy as Spycraft. Still, I've come to expect pretty good things from Chuck Rice as a games designer. I happen to be a fan of things a little more complex.
Which brings me back to Blood & Space. If I had to pick one RPGO product as my favorite, BnS is it. They do a number of other lines very, very well. But I'm limiting myself to one per blog entery (hehe). I ran a short campaign with BnS that I was particular fond of. I like the classes, the general mechanical improvements, and the general flavor. I never really got in to ship combat so that wasn't too much of a factor. But the d20 vehicle mechanics were actually pretty lousy--no fault to RPGO. Those mechanics were drafted by WotC. If you every have cause to use vehicle combat (or just about anything to do with vehicles) on a regular basis, pick up Spycraft and just steal that part. I may get inspired to write a long essay on vehicle mechanics... but not today.