My opinions;
iChat - supports chating, video conference, file transfer, and screen sharing with AIM, ICQ, MobileMe, gtalk, Facebook, and all other Jabber services. (my personal IMer.)
Microsoft Messenger:Mac - If your contacts prefer MS Live Messenger and Yahoo Chat AND/OR your company uses that MS IM server, (MS Messenger and Yahoo users can cross talk.)
Yahoo chat! - If your contacts prefer Yahoo Chat and MS Live Messenger, (MS Live Messenger and Yahoo chat users can cross talk.)
Skype - If your contacts prefer Skype. Just use the Skype official client.
Perian - It will enable Quicktime to play almost anything.
VLC - It will play anything. Also needed for DVD reading of handbrake
Handbrake - Useful to convert DVD to iTunes
Mac Office 2011 - most people like it for office apps, personally, (I find it difficult to work with and extremely over-priced. However, I seem to be in the minority. I perfer mail.app, contacts.app, iCal, + iWork)
Openoffice - some people like OO. The price is excellent. (As I mentioned, I prefer the Apple products)
Google Docs - some people prefer it. (I'm just not that cloud suave)
Firefox - If you like Firefox, (I use Safari on mac, chrome on anything else. Xmarks to sync my bookmarks between them)
chrome - If you like Chrome, (see above)
Xmarks - If you use more then one computer, Operating system, or browser it'll keep your bookmarks in sync
Dropbox - keeps a couple gigs of files synced across multiple machines, (windows, linux, mac, iphone, web.)
ClickToFlash - If you need flash installed, CTF will disable most of the junk pop-ups you'll come across.
Cyberduck - GUI (S)FTP client
iPhoto - awesome photo management (for personal use, not good for professional use)
keka - for uncompressing files in pretty much any format
Parallels Desktop - if you need virtual machines (you get sometimes get it on sale for really cheap, otherwise get VirtualBox)
VirtualBox - opensource virtualization software from Oracle
MS Remote Desktop Connection for Mac - best RDP software made for doing RDP connections to anything MS.
Smultron - for editing UNIX files
MacFUSE - for reading/writing other filesystems (you know if you need this)