Restaurant with a fun atmosphere

That's a great list StBloomSF! I would add Slider Bar (http://sliderbarusa.com) on Market at 16th, and also try Harvey's (http://www.harveyssf.com/) at the heart of the Castro at 18th and Castro for great drinks and a little more "local" flavor...

It all depends on what you mean by fun, but for my buck three of my favorites in Castro are:

1. Starbelly--for comfort food and cool laid back atmosphere
2. Canella--a pretty cool tapas joint
3. Bisou--a French place that is nice when you are on a double date or in a group.

But you really can't go wrong in the Castro--it is pretty much the capitol of fun!