Ask HN: What are you building on Google App Engine?

I was wondering what kind of apps/businesses folks were working on that are based on the App Engine. Would be very curious to hear what your experiences are like, especially about the limitations of the platform and how you worked around them.

I built a tile cutter (http://www.piritiles.com) app that'd let you instantly create a Google Map of any image a while ago, and now working on a more comprehensive map/mashup building app that I'll be launching in a couple months.

One of the interesting (and not very clearly documented) issues I've faced was the weird 500 errors the app would generate when you have consistent request response times over a few seconds. Technically all requests need to return in 30 seconds and the occasional ~10s response was okay, but for background processing intensive apps like Piritiles, using TaskQueue API (which was just coming out of beta at the time) was essential.