The one thing that most current augmented-reality apps have in common is that they're commerce-based: The nearest subway station. The nearest Subway sandwich shop. None of 'em want to teach you anything about your surroundings.
Cyclopedia begs to differ. It draws on Wikipedia entries — specifically, the 65,000 or so with geotagging info — then pops them up on your iPhone 3GS's live video feed. Starting with a default radius of 30 miles, the user can then zoom in as close to their current location as they desire. Near a location that Wikipedia lists, but doesn't geo-reference? Go ahead and update the article yourself! You can also search the whole of Wikipedia directly from the app.
There's a video on Cyclopedia's main page that gives a good overview of its capabilities. While you're there, check out Chemical Wedding's other iPhone apps, including a nifty director's viewfinder app that's worth an upgrade to the 3GS all by itself.
[Available for US$1.99 in the App Store]