Advanced CSS Prettifier: ProCSSor

Perform Structural Optimization of CSS Files: CSSO

 JS Tool To Scan Your Site and Show Unused CSS: Helium CSS

 Tool for Sorting CSS Properties in a Specific Order: CSS Comb

Free CSS Compressor/Compression Tool: CSS Compressor & Minifier

Find Unused CSS Selectors: Dust-Me Selectors Firefox Plugin

A CSS Redundancy Analyzer: csscss

CSS Validation Service

 Validate Your CSS Files: CSS Validator

CSS Validator Firefox Add-On

HTML / CSS Validator, Link Checker and a Lot More: Pendule Chrome Extension (Plugin)

Multi-lingual Web Tool for Indenting the Code: Tabifier

Create a Starter Stylesheet From Given HTML: Primer

Make Your CSS Beautiful, Consistent and Easy to Read: CSS Beautifier

Open Source CSS Code Quality Checker Tool: CSS Lint

Minify and Gzip Compress CSS & JavaScript

Open Source CSS Parser and Optimizer: CSSTidy

Compress and Tidy Up Your CSS: Devilo.us