Start off with a couple of useful looking gems:

bootstrap_forms - a way to work with twitter bootstrap 2

mailview - from 37 signals.  A way to see all the emails that your app generates in html preview.  I use letter-opener from Ryan Bates which is different as it opens the email as you interact with your app on dev

prickle - awesome DSL to simplify capybara syntax

Performance Monitoring

newrelic is your secret weapon - A nettuts+ tutorial on this now-standard third-party performance monitoring implementation

pssst... your Rails app has a secret to tell you - an article on 37signals' blog on how to get the performance data straight out your app without third-party tools like New Relic

Web apps and starting points

field guide to web apps - just what the title says...

hacker engine - a pre-build rails app incorporating stripe, bootstrap and kissmetrics.  $149 to get access to the github repo

sample app with Backbone.js and Twitter Bootstrap - looks like a decent starting point or project template

Speaking of Backbone.js...

Every time Ryan Bates spotlights something on Railscasts there seems to be an up-tick in attention to that something.  For those who saw Ryan's back-to-back Pro editions on backbone.js and want a few other introductory resources:

Thoughtbot's Backbone.js on Rails ebook

Backbone Screencasts - a series of 3 from Joey Beninghove that covers Backbone overview, Backbone with Rails and Backbone/Rails error handling.  I've picked up the first 2 and found them to be a really solid and accessible walkthrough of the subject

and of course, the Peepcode Backbone series offers another solid introduction