Ok, I can't believe someone submitted the Windows Key+E to launch explorer as a 'tweak', but I guess you'd be really happy to learn about it if you didn't already know.
In the spirit of higher learning, here's a few more:
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.
Have fun! And enjoy the cool tweaks that are here...