Resident Beer Guy
Honestly, not really. Safari and OSX have security advisories just like any other software, but they're patched quickly and I've never had any sort of problem with a browser hijack or anything even remotely similar.
Personally Safari rubs me the wrong way so typically the first thing I do with any Mac I use is install Firefox, but to each their own.
Some of my personal favorite software:
* Adium - great free multi-network chat client. Much less cutesie than iChat IMO
* Azureus - Java-based bit torrent client - works well on OSX
* MacTheRipper / DVD2oneX - dvd ripping/compressing software
* Roxio Toast - everything you could possibly need disc burning software
* Perian - Quicktime codec package that will play back just about any video file
x2 congrats on the mac purchase, took me about a week to fully switch over and i'm so glad i did