Adam's post about Teenagers from Outer Space got me thinking again about RPGs that only use a single die. If you take the idea of a rules lite game seriously then it seems to me you should go all the way and only use one die. Besides, the 1d6 roll has a long pedigree given that most wargames use a single die to resolve actions. There are actually quite a few 1d6 based RPGs around. Or at least there were. Several of them, like Bad Attitudes and Superteens, seem to have vanished into the Phantom Zone. But the ones that are still around are pretty cool.
Aside from R Tal's games ( the other being Dream Park) the only other commercial 1d6 based game I know about is The Dying Earth RPG. (Or maybe I should say was since it's now out-of-print.) It uses a clever mechanic where instead of rolling more dice you have a pool of points you can expend that allow you to re-roll your die. Very much a story-telling game, but I like the mechanic.
As for free games using just one die there's Zak Arntson's Adventures in Space!! It's a cool little game focused on space opera. The clever bit is that not only do you roll 1d6 for for character actions, you also have the options to roll the die to introduce plot twists to the game. Again very story-telling oriented, but very cool. There are a few others out there, like Simon Washbourne's Have a Go Heroes, but the one that really stands out for me is Michael Wolf's Warrior, Rogue and Mage.
It's a great fantasy RPG that uses an open-ended 1d6 roll allowing for higher throws. Check out Rob Lang's review for a good overview. It's a really well done RPG and the system could work in other genres as well.
Are there any other cool 1d6 based RPGs out there that I should know about?