istanbul: A Javascript code coverage tool written in JS

Yet another code coverage tool for Javascript, with the following features:

All-javascript instrumentation library that tracks statement, branch, and function coverage and reverse-engineers line coverage with 100% fidelity.
Module loader hooks to instrument code on the fly
Command line tools to run node unit tests "with coverage turned on" and no cooperation whatsoever from the test runner
HTML and LCOV reporting.
Ability to use as middleware when serving JS files that need to be tested on the browser.
Can be used on the command line as well as a library
Based on the awesome esprima parser and the equally awesome escodegen code generator
Well-tested on node 0.4.x, 0.6.x, 0.8.x and the browser (instrumentation library only)

Use cases

Supports the following use cases and more

transparent coverage of nodejs unit tests
ability to use in an npm test script for conditional coverage
instrumentation of files in batch mode for browser tests (using yeti for example)
Server side code coverage for nodejs by embedding it as custom middleware