Here are some resources that I have found useful.

Templates

• Bootstrap, a frontend framework from Twitter
• Ruthsarian Layouts, a collection of CSS templates with great compatibility
• Solucija's free templates

Other Frontend Resources

• Subtle Patterns, a collection of background textures
• slabText, a jQuery plugin for responsive headlines
• HTML5 Please, a compatibility guide for HTML5 features

Inspection tools

• Google PageSpeed
• REDbot

Misc

• httpstatus.es, a guide to HTTP status codes
• ViewDNS.info, a bunch of DNS tools