I have an external hdd full of media plugged into the USB port of my new Belkin router. What's the best way to beam that over to my Apple TV 2?

exactly what boxhead said.. Jailbreak your apple tv 2 and install xbmc

crazymorton is on the ball.

airvideo is the best - use it all day everyday

air video works however you will need to keep a computer on while watching the movie

Which basically means you use your main PC too much, or you should build a cheap server, which is less efficient than building a HTPC and offsiding it as a server on top for the rest of the house.

I like the first plan. It is a solution that is self contained.

The second solution basically suggests that you make the apple tv redundant.

Although i hope there's a second generation of these. I've got one but it doesn't like my TV. Sounds like a teething issue considering it works on all the other LG tv's i've tried it on.

I coupled that with a $10 bluetooth micro wireless usb keyboard/mouse combo. (which is the size of a remote)

Testing was good except for the whole it worked everywhere but my own TV. =_=.