A good overview on issues involved in scaling web applications -- focused on the "Prudence" framework, but more generally relevant as well.
A cloud-based Web site monitoring and load testing service
"distinct from most mainstream web server frameworks (and certainly most Python frameworks) because it is non-blocking and reasonably fast."
Various tutorials on topics including gzip, caching, code optimization, etc.
Yahoo's Exceptional Performance team has identified a number of best practices for making web pages fast. The list includes 34 best practices divided into 7 categories.
"New Relic RPM lets you see and understand [Rails] performance metrics in real time so you can fix problems fast."