I don't feel qualified to fully answer this list, but something I thing I should bring to attention of people is the GLOG (goblin laws of gaming), a D&D-based game (you can run a b/x module with them no issue) designed by well, a lot of people. The system is meant to be hacked and modified as you see fit.
Key feature are:
1: ease of use
2: low level/power play (every level you get "something" and at level 5 you retire... or continue, but only get nominal increases per level)
3: fast natural healing, so no "need" for a healer
4: spells don't have levels, very interesting system
5: every PC has a decent baseline fighting ability - a wizard stabbing a goblin with a dagger "works" (oh and daggers are good weapons)
6: SO MANY CLASSES - very easy to make your own, and there are amazing ones out there (
The best Glog classes). I've played a game where I was a antling petty sell-sword, along with a gun-priest and a monkey dad.
(edit: why is it so easy to make classes? because balance is not as important, and you only need to cover 4 levels)
The rules can be found in several places, but the most "orthodox" version is here: this one has a strong "feudal" aspect to it, but that isn't necessary if you don't like it
I've finally finished updated the original Rat on a Stick GLOG hack. Feedback and notes from playtesting was incorporated. Notable changes...
coinsandscrolls.blogspot.com
So what would I use the GLOG for?
For quick play and ease of learning and picking up
For one off-sessions
For dark and gritty
Want a low magic setting,
Player skill matters (there is an ethos in the GLOG: rolling is bad because it may fail! The best plans just work, no roll needed)
For the best player options and customization