App concepts don't get much simpler than this, or much cooler: Cyclopedia takes Wikipedia's tens of thousands of geotagged entries, and overlays them onto a live, compass-oriented view of your surroundings.
If this sounds an awful lot like Wikitude, the AR Android app that just can't seem to eek its way into the App Store, that's because it is an awful lot like Wikitude—except with a more polished interface, and, well, iPhone support. (Sort of: non-3GSers need not apply). As with Wikitude, firing this thing up around a familiar location won't really blow your mind, since Wikipedia's geographic article density is still pretty low.
But if you're visiting a new place—that is, a place Wikipedia editors care to write about—Cyclopedia ceases to be an AR tech demo, and with adjustable search radii, a top-down map mode and non-geocaches Wikipedia searching, actually starts to be useful. Available now in the App Store for $2; video demo at the source link. [Chemical Wedding]