I've written a blog with lots of documentation at http://www.gittr.com.
In addition, coworkers of mine at Citrusbyte have taken Sinatra and built a minimal, yet more full featured web-stack called Monk (http://www.monkrb.com). Basically it takes the Sinatra library and wraps it up with convenient conventions, libraries for testing, settings files.
I've used Sinatra for small projects like Irclogger (http://www.irclogger.com) (http://www.github.com/cschneid/irclogger). You can take a look at the code and see how simple it is underneath. I've also used Sinatra for larger projects involving many files, user login and authentication, database access, and more.
What you'll find when you're writing Sinatra is that almost nothing is provided in the way of helpers, but also that you won't miss them. You write your own helpers where you want, and use normal rubygems to provide other functionality. It's not really a web framework's role to render JSON when there's a perfectly normal JSON gem that does it well. On the other hand, various people have recreated the small helpful stuff in reusable forms, including fairly simple things like partial rendering, up to reverse URL lookup (like Rails provides person_edit_path(), etc).
I'm not sure what the other commenter is talking about with bloated memory footprint for Sinatra, it's a fairly minimal piece of Ruby code, and any memory bloat will be the fault of your code, and not the framework itself. A hello world app in Sinatra takes about 10 megs of ram to get up and running (under Thin in my case). Rails takes about 40 last time I checked. Everything from there is related to your app (be careful with ActiveRecord, it'll chew through ram).
To sum up: Sinatra is a do it yourself framework toolkit. It doesn't provide much beyond making it easy to build your own helpers and your own application specific code. If you need help or advice, swing by #sinatra on freenode.org. People are always around to help.