Every once in a while, an application won't launch because of insufficient memory, and even when you've shut down every other open application, there still isn't enough for what you want to do. Sometimes, the Mac won't let you do something (like run an installer) while applications are running, even though, so far as you can tell, none is. The problem is "invisible" applications. Not quite an application but a bit more than a mere extension, these are used by developers to ensure that particular conditions are present when running some applications. Unfortunately, they don't always quit cleanly, and they often take up more memory than they should (especially under Mac OS 8.5).

One elegant solution is Peek-a-Boo, which you can download from www.macfixit.com/library.shtml. This $US20 shareware product not only lets you see what invisible apps are running, it also tells you how much of the processor's time is being dedicated to each app. What's more, Peek-a-Boo lets you change the applications' priorities so that they can take less CPU time (this can often be safer than just quitting completely).