Samuel Dean at Web Worker Daily has compiled a nice list of applications that will make Mac OS X more familiar or recent switchers. Included in the list is Adium (for multi-protocol IM), WhatSize for identifying file sizes and types in a manner similar to Windows Explorer and Firefox, since they were probably using it on their Windows machine, anyway.
One tip I always share is the spotlight search feature in System Preference. Many switchers get lost in translation; that is, figuring out what Apple has named a certain system preference, or where it lives. System Preference search lets them enter a keyword, like "trackpad", and see the highlighted result. It's very handy.