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.<p>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.<p>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.
I built <a href="http://www.scoratic.com/" rel="nofollow">http://www.scoratic.com/</a> Its a daily list of the most valuable dropping domain names.<p>My biggest problem was ingesting data. The CSV file I needed to process was above 10MB and unable to be divided using range headers in my requests. This file size was too large to be requested by App Engine. I had to write a separate app on a VPS to process the raw file into a smaller and smaller more efficient file and then ingest that file to app engine.<p>At the end of last year, App Engine lifted this file size restriction and have been able to adapt the code and now it is full automated.
I haven't built anything worth bragging about, just a Zillow app playing with the Google datastore (cool stuff there).<p>Ran into the same thing with those 500 errors. I recall the error message seeming to me like it was a rare occurrence, not a Fail Whale "don't worry it happens all the time" kind of thing.
I've built <a href="http://www.tweethit.com" rel="nofollow">http://www.tweethit.com</a>
Basically it's TweetMeme for online products. Processing Twitter Streaming API, about 400k tweets per day.<p>Having some problems with failing cron jobs without any error messages though.