The godfather of Sinatra. Those who like Rails will probably want to start with Padrino rather than just bare-bones Sinatra DSL. Check out the blog tutorial.
This entire blog in Sinatra has about 200 lines of code and uses various gems. It is very simple because I like simplicity and I believe there are just a lot of things about Wordpress which I do not need.
Gems you might want to use:
An alternative to Paperclip for uploading images for Rails apps. It is framework-agnostic so you can use it for Sinatra apps.
Missing flash notices in Rails? You'll definitely need this one.
on your app:
post '/sing' do flash[:notice] = "Post has been created." end
on your views or layout:
<% if flash[:notice] %><%= flash[:notice] %><% end %>
Pony is used for sending email much like ActionMailer in Rails.
For clean url's. I have a lot of posts about google-friendly links and I wrote about friendly id which is used by nearly every Rails developer in the world. I do feel, however, the advantages of just using Stringex. Imagine having better queries if your slug isn't found on different table. It's a lot faster that way.
If any one of you have heard about Rails Plugins site which is dedicated to finding out which gems work for Rails3, I think there should be a site for Rubyists in general wherein Ruby programmers, Gem developers and users can discuss whether gems work for a specific framework. There are DSL's other than Sinatra. And for me there's nothing wrong with that as long as we have a way of knowing what works and what doesn't work for the DSL.