It is a web site by Sean Howard listing 300 game mechanics (well, currently about 60). I like them. It's like the ideas one occasionally has about game design, only with a neat formal twist, not to mention nice old school pixel graphics to illustrate them. I think it's a really good method for writing down ideas - it beats letting them moulder in notebooks or never writing them down in the first place. Making it a flat list of 300 and not claiming any kind of usefulness means you can judge the ideas in a different way than if someone said "Look look I have this cool idea that will make a million seller". Many of the ideas are interesting or thought-provoking.
(I feel tempted to steal the meta-idea... with credit of course.)
I also highly recommend reading Mr. Howard's webcomics. Especially IF Only..., a not-really-a-comic about interactive fiction (again). It has a running joke that had me in stitches. The other comics are fun too.