Mac App Store is Live
You'll need to be running OS 10.6.6 which is available through software update (Go to the Apple in the top left corner of your screen, click 'Software Update' and you're off to the races).
After a reboot you'll have a new app icon sitting in your dock.
Initial thoughts: I can't wait until the next piece of software I need to help my dad install and I can just send him a link and say "buy this." Definitely makes life much easier to buy and install applications on a Mac.
Recommended first apps: Twitter for Mac is free, The Incident is a great little game that I play on the iPhone as is Angry Birds. The really great thing is now you can buy individual apps within Apple's iLife and iWork suite - so iPhoto is only $14.99 and you don't have to get iMovie or Garageband if you don't ever use them.