lets see- crossover for running windows programs (belive me you don't need boot camp or parallels)
AirRadar - if you you are getting a laptop
Utorrent - I think we all know what this does

Limewire - Again

Frostwire -

GoogleEarth - A must have
Boxee - If you like tv shows and movies check it out. It is a really cool program.
AppCleaner - A must have. This should be the first thing you download.
DockLibrary - You wont use this at first, but in a month or two you will use it a lot. If you want to customize your dock this is a good one.
CandyBar - Again you wont use it at first (you will be to amazed about the apps that come on the mac) but in a month or two you will want this. It customizes your dock and more importantly your icons.
SuperDocker- Same thing as Docklibrary but it is good to have both.
Rightzoom - google it. it is very useful.
Burn - Very useful for burning dvds and iso's
Here is a invaluable website for any mac begginer
http://web.mac.com/simon_elliott/simon_elliott@mac.com/Software.html
I hope you enjoy.
Also Google all those programs to find out how to download, and because some of them are paid apps, search youtube on how to get it for free

You might even see some of my videos.