IrfanView Shell Extension is an incredibly simple but useful freeware Windows program. It extends the excellent IrfanView image tool so that you can quickly do some of the more common operations, such as file format conversions, directly from the right-click context menu of Windows Explorer.
My favorite use of it is for JPEG screenshots for bug reports and blog postings (because BMPs are too darn big). I hit the Print Screen key and then create a new .BMP in Windows Paint and paste with Ctrl+V, save the .BMP file, right-click it and select IrfanView | Convert to JPG.