If you share your Mac, chances are you're familiar with the concept of fast user switching, which allows users to switch between accounts without closing applications and logging out of either account. Free, open source application WinSwitch is a fast user switching enhancement that optimizes the display and launches apps on switching.
The default fast user switching feature displays your name in the menu bar, which is a terrible waste of menubar real estate, especially if you've got a long name or a lot of menubar apps; WinSwitch displays your user picture. WinSwitch also launches items found in the Switch-In/Switch-Out folders when you switch users, allowing you to define actions to take place when account switching is underway (the sample items they provide tell iTunes to start and stop playing, but any document or app could be launched). If you share your Mac with any other users, you can surely see the potential. WinSwitch is free, Mac OS X only.