[mark_l_watson]

AppEngine is pretty much awesome when you want to reduce web portal hosting costs. I am rewriting/augmenting a Rails web app in JRuby + Sinatra + DataMapper so it will run well on either Java AppEngine or one of my own servers using (C)Ruby. If my project gets a lot of users I would like to keep my costs down. (I usually deploy to Amazon AWS, but if you can live within the AppEngine restrictions, then AppEngine is even cheaper than Amazon.)

The ability of receiving email sounds good, but I usually only use this for new account verification - with AppEngine you can just use Google accounts.