You're probably going to want to get another internet browser in addition to Safari for that occasional page that won't load in it.
There is Firefox of course, but if you don't need all of the plugins I would perhaps reccomend Camino or Flock. Both are a little more "mac-like".
I particularly like Opera as well, althoug it often suffers from the same non-loading pages as Safari.
Not too much out there are must haves IMO. There are more specialty tools that come up as you might need them. Cyberduck is great, but if you don't do FTP then there is no point. Stuffit is good if you need it for extracting zrchives besides .zip. If you have a particular hobby there is almost always software for it. For example I really like to cook, so I am looking at recipe management software like MacGourmet.
If you need time/project management tools look into Quicksilver, Growl, iGTD, Journler, Yojimbo or Delicious Library. All very cool applications for managing information on your Mac.
Seashore or Gimp for image manipulation.
Your interests guide the must haves more then anything, because reall mac comes with most of the foundation tools you need from photo management to DVD creation to email.
Neooffice I think is fantastic for really doing reports and formal writing, but if you aren't in school anymore, or working in a field that requires that, then I think you might have more flexibility with iWork. Pages really offers some more creative formatting for your work, and if you are doing things like sending 4th of July invitations to friends, or making a garage sale flyer then the $70 spent on iWork is well worth it. It is much more a Page layout program then straight up word processor.
The only other must haves would be the Perian and Flip4Mac plugins which will enable Quicktime to play just about any media file. Get those installed and you won't have to worry about downloading any additional programs to play media.
Also Google Earth is always a blast to have and comes in more handy then you might think for finding businesses, resturants, driving directions or just interesting information.
I would reccomend checking out the following two sites
http://www.opensourcemac.org/
http://freewaremac.net/