Windows only: Free application Minimem reduces the memory usage of individual running applications on demand. Similar applications which promise to make your computer run faster by freeing up RAM are a dime a dozen, and many of them aren't all they're cracked up to be. After giving Minimen a try on my PC and reading FreewareGenius's detailed review (along with the author's comments on that post), Minimem appears to be the real deal under the right circumstances. Minimem removes unnecessary memory pages from running processes you tell it to optimise. The program isn't the most useful for applications that already have great memory management of their own, but it seems to work well on many applications—both small and large—that have a larger footprint than they should.
I tested it with Microsoft Outlook and Firefox 3 and saw good results, and the FreewareGenius review also saw positive results with Digsby, Internet Explorer, and VolumeTouch. Minimem is freeware, Windows only, requires .NET 3.5. If you give it a try, let's hear how it worked out for you in the comments.