Castles and Crusades is great if you like the general idea/mechanics of D&D but want something simpler (much simpler). It uses many of the same mechanics and conventions of D20 (same attributes in the 3-18 range, HP, AC, roll high on a D20), but also has some of the feel of AD&D (class abilities instead of feats and skills, less tactical combat). Character creation takes about 10 minutes, and it is very quick in play.
D6 is my favorite system. It is also very quick in both character creation and play while allowing for a decent amount of depth and customization.
Savage Worlds I have read but not played. It is pretty easy, but it has a few more fiddly bits than D6. I know a lot of people use it as their alternative to D&D.
True20 is a good game. It isn't really lighter than D&D, but it is more streamlined and runs a bit more smoothly. It goes the opposite way of C&C, it is all feats and skills, with almost no class abilities. The three classes determine which feats you have access to and your saves, and not much else. You build whatever class you want with feats (many of the class abilities from D&D are feats in this game), so if you want a wizard with barbarian rage, you can do it.
In short, they are all good systems.
