If you were one of the few people to go out and buy an original AppleTV when they came out a few years ago, then you not only paid a lot of money, but you also likely now have a neglected Apple logo adorned paperweight. Sure, it still works in its current state, but Apple isn't updating the software anymore, so you won't get the fancy updates like its newer and cheaper sibling. So what do you do with old tech? You hack it, that's what. And in the process, you might find that original Apple TV still has a few uses yet...
Flash It: There are numerous options for reflashing your old AppleTV, but the basic premise is that it adds functionality by customizing the operating system. ATV Flash is one option, which allows you to surf the web, play more formats than you can get into iTunes, and expand your storage capacity using a USB hard drive.
Swap Out the Hard Drive: This is by no means a quick fix nor for the faint of heart. But if you want to add some extra storage, but keep your AppleTV factory fresh, you can do so by upgrading the hard drive with a more spacious PATA hard drive. There's an excellent writeup on the process over at Macworld, but take note, it's not quite a beginner project, requiring both the right tools and some Terminal based command line work.
Add AirPlay: The original AppleTV hasn't been updated in a bit, so there's no option to add AirPlay. Remote HD changes that, allowing you to stream audio and video from your iOS and Android device to your old, but not forgotten Apple TV.
Install XBMC or Boxee: If you want lots of versatility and a pretty nice open-source option, consider installing one of these two popular pieces of software. Great instructions are over at the XBMC website and Ars Technica, but here's the gist: you'll get a completely customizable interface, and a media center that puts the new AppleTV to shame.