You'll probably want some sort of "office productivity" software - word processor, spreadsheet, &c. Appleworks is an inexpensive alternative; Microsoft Office is probably the best if you have the budget for it. I use an older version of Microsoft Office.
I personally couldn't live without CodeWarrior, my development IDE, but nonprogrammers wouldn't need this.
I have a bunch of other useful little applications I've accumulated over the years, but none of it is really in the "essential" category.
The main tip I have is this: if you can't find a feature that you are used to on your prior platform, spend some time looking for it. It's probably there, but it may be just a little different - and to a new user, the little differences can be hard to figure out. Don't be afraid to post in the Mac forum here, or on other Mac sites. Within a few weeks, you should be working more efficiently than ever.