Microsoft PowerToys for Windows XP have been a godsend for many of us. Unfortunately, they don't work so well with Windows Vista or Windows 7, but thankfully Brice Lambson has created a clone for Windows Vista of the Image Resizer powertoy.
http://www.codeplex.com/ImageResizer
- or -
http://sourceforge.net/projects/phototoysclone/
The original Windows XP PowerToys can be found here:
http://www.microsoft.com/windowsxp/Downloads/powertoys/Xppowertoys.mspx