What exactly are you looking for? A complex Sinatra app? Sinatra is largely suited for smaller apps, so you won't find many super complex apps built on it, though there are some open source examples. However, it is used rather extensively in conjunction with large, high traffic Rails apps. For instance, the RubyGems gem sever: https://github.com/rubygems/gemcutter/blob/master/app/middle...

If your requirements fit within what Sinatra can provide, you shouldn't hesitate using it. It's a very straightforward framework and works well. If your requirements are more complex, look at Rails or Padrino, the latter of which is built on top of Sinatra and is actually pretty awesome.