The Sinatra Web dev framework has been around for a while, but I've recently been seeing it popping up a lot more.
Simon Willison, the co-creator of the Django Python Web-dev framework, even mentioned it to me at SXSW Interactive when I buttonholed him after his panel on the Changing Face of News, to ask him what the Guardian is using for their API service.
(Sadly, he said, it's still all enterprise Java. NPR is using a mix of PHP and Java, and the New York Times runs their API service on Ruby on Rails.)
@wycats sinatra is filling the gap now that merb is an organ donor ;)
@wycats I see some people taking to Sinatra now that Merb isn't the angsty choice. But mostly, its people looking to do more with less code.
Unless you have a case of man-with-hammer syndrome, you see that no single tool is right for every job -- and Sinatra's lightweight approach (it doesn't even bill itself as a framework, just a 'DSL') looks great for simple uses where you don't need all the overhead of Rails (or even Merb).
I recently converted the Fleegix.js project site from a Merb flat app to a gorgeously minimalist Sinatra app, and found Sinatra's simple aesthetic to be just about perfect. I could also see using it for throwing up simple Web services as well (although it's hard to beat Merb's resource routes and braindead-easy display/provides content-negotiation for that).
My next Sinatra project will be the EpiphanyRadio site, which is definitely due for a little love. It's another simple site (currently a bunch of raw mod_ruby/eruby) which would be a lot nicer as a simple Sinatra app.