I've run across a few neat little utilities for OS X that I've used for a while and like and one that didn't work out as well.
GMailStatus is a little utility that puts a count of messages in your GMail inbox on the status bar. If you use GMail, its nice to be able to see new mail without popping out to the Web. One thing I've noticed is that it tends to use a lot of memory, which I don't understand. There may be a memory leak in the thing somewhere.
From time to time, I'll plug something into my Firewire or USB port and not be able to see whether the system recognized it or not. Or my network cable comes loose and I lose connectivity. Granted Software's Peripheral Vision is a little utility that flashes the name of devices as their plugged in or out of the computer. I haven't used it, but there is also a way to launch scripts on these actions as well. This is handy enough that it ought to just be built into the OS.
I also tried a plug-in for Apple's mail client, Mail.app, called Mail.appetizer that flashes a summary of newly arriving mail on the screen over the top of other windows. I tried it for a few days and then disabled it. I was reading mail twice and I don't even like reading most of it once! The problem is that I like to delete some mail and send some to junk and Mail.appetizer doesn't support the latter.