Lets face it, unless you have a photographic memory, no developer can remember all the different functions, options, tags, etc. that exist. Documentation can be cumbersome at times, thats why I like cheat sheets. They are quick references that feature the most commonly forgotten things on a specific topic. You can print them out and hang them on your wall, or just keep them handy in your bookmarks for quick reference.

I have rounded up over 30 cheatsheets that developers might find useful.

Web Development Cheat Sheets

• JavaScript
• CSS
• CSS
• Hex Codes
• HTML
• XHTML
• HTML Entities

Databases / SQL Cheat Sheets

• MySQL
• MySQL
• Oracle
• SQL
• Sybase

Language Cheat Sheets

• ColdFusion - work in progress
• PHP
• Ruby
• Regular Expressions
• JSP
• Java
• Python

Version Control Cheat Sheets

• CVS
• Subversion

Other

• Google
• Windows
• Unix
• Unix
• Unix
• Vi
• Vim
• mod_rewrite
• ASCII Character Codes
• htaccess

Commercially Printed CheatSheets

• Visibone - javascript, browsers, colors, fonts, html.
• TechnoChallenged - web dev, programming, os, software

Am I missing any good ones? Feel free to post any others in the comments.

Updates:

• Perl - thanks Dom.
• Regex - thanks bjhanifin
• Perl - thanks Derek
• Subversion - nice reference card, thanks Andreas
• Debian - thanks again Andreas
• UML - Andreas again, thanks
• PostgreSQL - thanks for the suggestion frank.