Ruby and Sinatra make it ludicrously easy to make a webapp, but getting started, as with any new language or framework, can be daunting. By the end of this part, you'll have a simple template that talks back to you!
Some of these are obvious (i.e. get exceptional, hoptoad), many of these aren't Ruby-specific, but I thought it might be nice to put all these in one place, at least for my sake.
there's github, of course, if you're already using that...
16bugs limit: 1 project
I'm not including full hosting platforms, like Google Code here, but you could use those, too
One-off emails (i.e. signup)
Sendgrid limit: 200 emails/day
GMail allows 500 emails/day, but doesn't offer all the doodads that Sendgrid does
Mailing list (i.e. newsletter)
Mailchimp limit: 500 subscribers, 3000 emails total a month
User Avatar Hosting
Gravatar limit: none
RPX limit: up to 6 providers, instead of 12
Harvest limit: 2 projects, 4 clients, 1 user
Help desk thing
Just kidding, couldn't find any free hosted help desk apps
Anything else I'm missing that every webapp needs?
Update 10/5/2010: added Tripwire