I have this weird situation crop up every once in a while.
My OS X (Snow Leopard) system will simple stop being able to do anything "new". It can't start any new processes (like, for instance, a new app or a new bash process) which it hasn't loaded before. Things that are already running may be able to start again... but if it's out of cache (maybe?) it just sits there hung.
Eventually the whole system degrades, app by app, process by process, to the point where nothing responds (including Finder, UI, etc.) and I have to do a hard power cycle.
Any ideas? When it happens, I can't even start Activity Monitor or
top if they aren't already running, so it's hard to pin down.