I was browsing through amateur radio blogs today, and saw this post by K0NR, which mentioned a site I hadn't noticed before by KC5VQD: callsignbytxt.org. I tried it out with my own call, and other than telling me I was 15 miles from myself (something the site warns may be wildly inaccurate), it was quick and worked perfectly. I've found myself writing down or trying to remember calls I'll see on license plates and whatnot, and this is a great way to stop forgetting them before I can look them up.
He's even nice enough to include a RESTful API, returning in JSON or XML. Beautiful.