Picasa 3 automatically saves screen captures without requiring any user interaction — hit PrintScreen, capture gets saved. Hit it again, another capture is saved. For me that works best, because I usually need to capture a series of screenshots which I would crop later if necessary. No BS, just gets the job done.

Built-in editor, big number of formats to chose from and – what most programs definitely lack – you can see projected size of image to save with particular settings, and preview window to see how good it will look like.