We've been using this on several Macs for years now. It's so valuable that it's set to automatically start on login for all our users. If your machine starts to get into trouble with an application hogging too much memory, you will see it immediately. The window can be formed to take up a tiny sliver of space, yet still give a complete graphical view of memory usage. It is particularly good for less technical users, yet shows details useful to sophisticated users as well. There seems to be no hit on system resources whatsoever.

We've never had the first problem with this little application and the price is right - free.

It's indispensable for us. Do remember, this is PPC only.