Just got my first Mac

Adium ( http://www.adiumx.com) for your instant messaging needs.

http://www.perian.org/
http://www.microsoft.com/windows/windowsmedia/player/wmcomponents.mspx

^ Those will beef up QT Player so that it can play most media formats out there.

http://www.videolan.org/vlc/

^ That is for the few formats that you still can't play ;)

You'll want to get the Mac version of HandBrake (http://handbrake.m0k.org/) for DVD ripping and iSquint (http://www.isquint.org/) for converting everything else for your iPod.

You may also be interested in Doug's "Set Video Kind of Selected (http://dougscripts.com/itunes/scripts/ss.php?sp=setvideokindofselected)" AppleScript if you're into adding TV Shows to iTunes. It will let you batch edit the TV Show tags. You'll want to bookmark this site, if there's something iTunes won't do for you, chances are you'll find an AppleScript here that will make it.

There's also TVShows (http://tvshows.sourceforge.net/) for subscribing to TV Show bittorrents. It won't download shows, but it will send the new episodes over to your favorite bittorrent program for downloading.

Since I believe they have improved Spotlight quite a bit in Leopard, there may not be much of a need for Quicksilver (http://www.blacktree.com/), but this gem shouldn't be over looked. I have yet to use even a fraction of it's potential, but it is a great App launcher and does way more than I can figure out how to use it. It creates a log of the most used apps, documents, folders and more. Hit the CRTL and the Space bar, a box pops up, type a few letters of the program you want and usually the program will pop up in 1-2 letters, hit enter and it's loading. The same happens with folders, URLs and more. I haven't even used a tenth of Quicksilvers capabilities.

Not an app tip, but something every new Mac users should know is how to really secure your system. I would recommend hitting System Preferences/Accounts and creating a new account set to "Allow user to administer this computer", this is your Admin account, remember the password. Once you have done this remove the check from this option in your main account, this will set it as a limited user. This helps prevents any changes to your system with out a popup that requires you to authorize the changes. This is what makes OS X and every other Unix/Linux OS secure and not requiring aintivirus or spyware software yet. And if you are not using a router, make sure the firewall is turned on in System Preferences/Sharing as well. I hear they forgot to turn the firewall on in Leopard.

MPlayer OS X.
I like it a little more than VLC

PearLyrics, for all your lyrics needs (better features/accuracy than others)
um... I can't think of any others, but I might post back