As someone who just recently bought his first ever Mac a couple weeks back (I went with the iMac, too), allow me to share what I've done so far...
1.
http://www.londondrugs.com/Cultures/...7&ProductTab=3
$199 for a 500GB backup drive that uses Firewire800. It's made by Iomega, and it comes out of the box ready to use with Time Machine, too. If you don't already have a wireless router, you can look at the Time Capsule from Apple (wireless router with built in hard drive), though everything I've read says to stay away from them. They are (apparently) bad for bogging down your network with their hourly backups. I use the Imoega one I linked to, and it works awesomely. Well, technically, Time Machine works awesomely, but it's tough to beat a 500GB external with Firewire800 support for $199. Apple sells them on their site, too.
2.
http://www.macupdate.com/info.php/id/21430
SimplyRar. Think WinRar for Mac.
3.
http://www.transmissionbt.com
Transmission. Thin uTorrent for Mac. (though I wish there was a uTorrent release for Mac)
4.
http://www.microsoft.com/mac/product...8/default.mspx
http://www.neooffice.org
http://www.openoffice.org
Office Suites for Mac. Two are free, one is not. Out of the free ones, I prefer Neo Office. Having said that, I'm actually using the M$ offering.
5.
http://www.videolan.org
VLC is an all-in-one player you may have already used with Windows. It's even better on a Mac.
Edit:
Also, about the mouse/keyboard...
The Apple Wireless keyboard sucks. It's a laptop keyboard. This means no keypad and very very small. Get the wired one, it's small too, but it has a full set of keys (both a backspace and a delete key, for instance). It also has 2 usb ports - which are almost mandatory, since the iMac istelf only has 3 in total.
The Apple mouse (wired or wireless) sucks. If you're a Windows (or even Linux) user, then you like to right click. The problem with the Apple mouse is that, even though you can set it up so it recognizes right clicks, you have to lift your left finger completely off the mouse, or it will think it's a left click. Sounds weird, but try it and you'll see what I mean.
I ordered both the wireless keyboard and mouse from Apple with my iMac. I then had to buy the wired keyboard and a wireless logitech mouse from Future Shop afterwards.
Other than all that... I hope you got the 24" one and I also hope you didn't upgrade the RAM from Apple. You can get the same RAM from Crucial or Mushkin, etc... and save yourself about $100.
PS: I absolutely LOVE my Mac.