Arcane Heroes is possibly one of my favorite examples of an ultralight game. It doesn't have a whole lot of frills, but it has substance where it matters (though I would recommend expanding it and adding examples, just for the sake of 100% clarity [does the bonus apply to dice rolled, or the wounds inflicted during combat?]).

It's also an exceptional example of how to do typesetting- simple text boxes in an interesting but not overpowering background with interesting fonts to add flavor and a clean font for reading. Also, the lines of text in each column match the other, so there's no obsessive urge to make them meet each other.

If you're looking for one of the best free games in a long time, look no further than Arcane Heroes.