Windows only: Image resizing tools are a dime a dozen, but free utility Shrink Pic is actually an extremely clever original: Instead of any requiring manual processing, it runs in the background and automatically resizes images whenever you attach or upload them.
A perfect tool for frequent Facebook uploaders, for example, Shrink Pic works with a number of applications (most browsers, Outlook, Thunderbird, Skype and MSN Messenger, to name just a few) to monitor when you upload, attach or send pictures via IM. When you do, it automatically resizes the images in the background based on user-defined settings, then uploads the smaller image (so you don't have to wait several minutes for an upload to complete just to have it severely compressed at its destination anyway). It can even resize multiple photos in the same upload. The speed at which it resizes and the quality of the compression are nothing to sneeze at, either.
Shrink Pic saves the resized images in a temporary directory, so your originals are never touched — just copied. You can use any kind of compression level you want, as well as choose from five different photo types to check for. If you want to disable it, all you need to do is uncheck an option in your system tray — and re-enabling it is just as easy. You can even install a portable version to a USB drive, so you never have to resize images again — not even at other computers.
Shrink Pic is a free download, Windows only.