Seems like you could do this just with some AppleScript. Doesn't OS X support running a script when a file is added to a folder. There's a name for it too...

Sounds like you need Skitch, which does this and a lot more. It's one of my five must-have apps.

http://www.skitch.com/

What are the other four?

For screenshot sharing there's also GrabUp, which is simpler and quicker than Skitch but doesn't have any editing features.

I should have been clearer: it's one of my top five productivity apps. The other four would be:

Textmate, Omnifocus, Balsamiq Mockups, Yojimbo

Very clever combination of two of my favorite utilities.