An interesting change in Leopard was the addition of Data Detectors to Mail, which automatically detect info such as dates, times, names, and addresses, and let you use that data directly from the message in iCal. It can be a very convenient feature, but some users find it annoying. To disable it, quit Mail, and in Terminal, simply enter:

defaults write com.apple.mail DisableDataDetectors YES

If you want to enable it again, quit Mail, and repeat the above command, but replace YES with NO (confounded double negatives!).

[robg adds: This previous hint explains how to enable Data Detectors in iChat, which are disabled by default in that program.]