A peculiar problem I have faced since upgrading to Mac OS X 10.5 (Leopard): mouse clicks suddenly stop working! I can still move the mouse around and use keyboard navigation, but clicking any of the mouse buttons results in no action. The trackpad on my MacBook stops working too. After killing processes one at a time, I discovered a workaround: killing Universal Access (you will have to use the terminal to find the process id, in order to kill it; you could probably find the process using Activity Monitor also, perhaps) caused mouse clicks to start working again.

Not sure what/why is going on and this is a poor fix for any user/application that requires services provided by Universal Access. But it at least helps me keep working!