A useful tip for the Mac OS X terminal..
..because I keep having to tell people about it.
If you like your terminals to be white-on-black, like me, you'll get annoyed, because when two terminals overlap, there's no visible line between them. Set your terminal background colour to ~3% grey, instead of pure black. You'll be able to make out the shadow of the upper terminal overlaid on the lower, which serves as a very good border, but the contrast in the terminal will still be very strong, you'll never notice it's not really black.