The more I fiddle with Microlite20, the more I like it. It's an extreme simplification of the 3.5 edition fantasy system that lets one use most of the interesting material (adventures, monsters, etc.) with little effort, but without all the work or most of the annoying features like battle mat combat and feat chains from hell. The basic rules are 2 pages. Microlite20 is VERY easy to modify to fit your view or your campaign world -- much like Original D&D was.
Combats are abstract and fast so they don't get in the way of the adventure. Combat-oriented adventures can fit in many more combats in a 4 to 6 hour session and non-combat-oriented adventures can still have a few battles without taking up lots of time needed for roleplaying. Like older versions of D&D, Microlite20 does not have long lists of skills and feats. Instead, players are free to have their characters try just about anything the DM considers reasonable without needing to have a specific skill or feat. It's a very nice system. The main Microlite20 site (which is unfortunately down a lot) has a large number of user-written options and alternative rules. This site has copies of the core rules and early/major expansions if the main site is down.
There are several interesting threads at EnWorld (long original thread, second thread) and at RPGnet. These threads show how easy the system is to use and modify.
I think I'm in love with a set of RPG rules for the first time since the Mentzer boxed sets of D&D came out in the 1980s. (Before that it was Original D&D in 1975).