Windows Key + F - Launch Find tool
Windows Key + M - Mimimize all open windows
Windows Key + R - Launch the 'Run' button
Windows Key + U - Launch the utility manager (win2k & XP only?)
Alt + Tab (Hold Alt & Press Tab repeadly) - Task Switcher
Alt + F4 - kills the 'active' application - fun(?) for killing web popup windows quickly when they are coming at you like spam madness!

Ctrl+Alt+Del - In Win9x/ME (why are you using that?) it launches a 14m3 version of NT/2K/XP's Task Manager. In NT/2K/XP you get a menu with useful options for lock computer, logoff, shutdown, change password, task manager etc.