Incredibly useful for developing static sites. Runs an http server on the command line and processes your LESS, SASS, etc. on the fly. Also has compilation for when you want to compiled build to deploy to S3 or similar.
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This is my go to tool for building static sites and prototypes. Built in CSS compiling, HTML includes, templating languages, it has it all.
I'm surprised this isn't more popular - an incredibly light server with great precompiling. I'm using it as a personal documentation server for my own Markdown files, and it's also inspiring our own open source project. Great founding team too.
SeenHunt for Chrome
Corilla