This is one of my favorite applications for Mac OS X, and it was only available for PowerPC because of major differences in memory stuff on a low level. Rogue Amoeba has come out with a beta version for Intel.
It shows, in the menu bar, the memory usage of the foreground application. For example, right now I can see that Firefox is using 76.7 MB of private memory. Link to the Intel beta page here. If you want the PowerPC version, go here. It’s at the bottom of the page.