Savescreenie is a free, user-friendly front-end to access an undocumented feature in Mac OS: namely, the ability to save screen captures (typically grabbed with Command+Shift+4 or Command+Shift+3) in a variety of formats, instead of just the default PNG format (or PDF, if you're using Mac OS 10.5).
When you open Savescreenie, you're given a list of 10 formats to choose from (PDF, PNG, JPG, JP2, TIFF, TGA, BMP, PSD, PCT, and GIF), or you can reset the format to the default for the Mac OS you're using. The app also includes links to general information on the Web about each format. In addition to changing the default format for screenshots, Savescreenie lets you change the default save location and the default base file name (which is automatically extended with the date and time). That's it.
Many users will have no need for Savescreenie--and those who do might just use it once--but for anyone who works with screenshots a lot, SaveScreenie is a great little tool.