If you leave your Mac on for long periods or if you browse a lot of pictures for example, the Finder will start using more and more memory and if you run top you’ll see that the resident memory is very high. To do a quick release of memory (sometimes over 500MB) just do Alt+RightClick on the Finder icon on the Dock and hit Relaunch. Usually Finder will go back to using 20-30MB instead of that half-gig.