I just installed Mac OS 10.4, Tiger, this morning, and found some interesting tricks. I'm sure there are tons more, so if you find something fun or useful, post it here!
1) Dictionary trick: Hover the cursor on your mouse over a word, then hold down CNTL-CMD-D, and the dictionary will find the definition for you. So far I've found this to work in Mail, Safari and iChat (and I'm sure there are others). I'm going to hotkey this one!
2) When you select "Current iTunes Track" to display from the Available menu in iChat, other iChat users get an arrow to click on to take them to the iTMS to preview or purchase the song.
3) Dashboard Widget trick: If you want to keep a widget on your desktop instead of having to F12 the lot of them, there's this:
-First enable the dev mode by opening the Terminal and typing:
defaults write com.apple.Dashboard devmode YES
then press Return, logout and log back in again.
-Then activate Dashboard by pressing F12.
-Begin dragging the widget.
-Press F12 again, before letting go of the mouse button, and drop the widget wherever you want.
You can repeat this for as many widgets as you like, and move them around your desktop. Note: the widgets will be on the top layer, and obscure anything beneath them, so choose wisely! I've a fairly large screen, so to begin with I've just put the Flight Tracker and iTunes widgets on the desktop so far, and there's plenty of room for everything else I need to see.
To UNDO this, just repeat the process, dragging the widget back into Dashboard.