Upshot appears in your menu bar and when you want to take a screenshot, you can click on the Upshot icon and select Take Screenshot. Your cursor then appears as crosshairs, and you can select an area of the screen that you want to capture. You can also set a time delay, as well as a hotkey combination.
Once you've taken a screenshot, Upshot can automatically upload the image to Image Shack, Imgur, TwitPic, or a FTP server. You can also use custom scripts.
Upshot is $15 and is compatible with OS 10.5 or later. A free trial is available for download.