I've been running the freeware app MemoryStick for a few days. It places a small GUI on the desktop that shows wired, active, inactive and free memory amounts.
I have read that even under OS X applications can hold onto memory and if the Mac isn't restarted regularly you can actually get to the point where much of the memory is tied-up in one way or another.
Well, that certainly has been my experience. I have 2.5GB of RAM in my G5 iMac and even though I wasn't running any memory-intensive apps my various small apps that I leave open much of the time were claiming a lot of RAM, to the point where I had relatively little inactive and free RAM remaining, along with 6 swap files open. MemoryStick also indicates when page-outs are occuring.
In the past I always turned-off my Mac at the end of the day, but recently I have been putting it to sleep and I turn it off maybe once a week.
After restarting the situation was reversed... Even though I have most of the same apps open, I have a lot of inactive and free RAM, and relatively little RAM tied-up.
The
MemoryStick Help file has some good info on how OS X manages memory usage.