A site with cheat sheets for developers covering a large number of different technologies:

Programming & Scripting Languages:
ActionScript :: Ada :: AJAX :: C :: C# :: C++ :: CFScript :: Clojure :: COBOL :: ColdFusion :: Common Lisp :: Delphi :: Erlang :: F# :: Fortran :: Haskell :: Java :: JavaScript :: Lisp :: Lua :: MATLAB :: Objective-C :: Pascal :: Perl :: PHP :: Prolog :: Python :: R :: Ruby :: Scala :: Scheme :: Server Side Includes (SSI) :: Tcl :: VB.NET :: VBScript :: Visual Basic (VB)

Markup Languages:
CSS :: HTML :: JavaServer Pages (JSP) :: Markdown :: MathML :: Schematron :: XAML :: XHTML :: XML :: XPath :: XSLT

Query Languages:
LINQ :: PL/pgSQL :: PL/SQL :: SQL :: T-SQL :: XQuery :: Yahoo! Query Language (YQL)

Libraries & Frameworks:
Adobe Integrated Runtime (AIR) :: ASP :: ASP.NET :: Blueprint CSS :: CakePHP :: CodeIgniter :: Django :: Drupal :: Enterprise JavaBeans (EJB) :: Fusebox :: GNU Readline :: Google Web Toolkit (GWT) :: JavaServer Faces (JSF) :: Joomla! :: jQuery :: JSTL :: Kohana :: Lift :: Lime Testing Framework :: Microsoft .NET Framework :: MODx :: MooTools :: OpenGL :: OpenSSL :: Oracle ADF :: Papervision 3D :: Prototype :: Ruby on Rails :: Scriptaculous :: SimpleTest :: Smarty :: Spring :: Symfony :: Vaadin :: Yii PHP Framework :: YUI :: Zend Framework

Operating Systems:
BackTrack :: IBM AIX :: Linux :: Mac OS X :: Microsoft Windows :: Unix

Version Control:
Bazaar :: CVS :: Git :: Mercurial :: Perforce :: Subversion (SVN)

Servers:
Apache :: .htaccess :: IIS

Databases:
DB2 :: Informix :: Ingres :: Microsoft SQL Server :: MS Access :: MySQL :: Oracle :: PostgreSQL :: SQLite :: Sybase

ORM:
ActiveRecord :: Doctrine :: Hibernate :: Propel

Blogs:
Blogger :: Movable Type :: TypePad :: WordPress

Websites:
Ask.com :: Gmail :: Google :: Google Adwords :: Google Docs :: Google Maps :: Google Reader :: Twitter :: Yahoo!

Text Editors:
ed :: emacs :: E Text Editor :: TextMate :: Vi :: Vim :: Vimperator :: x-emacs

Applications:
3D Studio Max :: AAV ColorLab :: Adobe Acrobat :: Adobe Dreamweaver :: Adobe Fireworks :: Adobe Flash :: Adobe Framemaker :: Adobe Illustrator :: Adobe InCopy :: Adobe InDesign :: Adobe Photoshop :: Adobe Photoshop Elements :: AppleWorks :: Blender :: Capistrano :: Cinema 4D :: Eclipse :: Fgdump :: Final Cut Pro :: Front Row :: GNU Debugger (GDB) :: Hping :: iCal :: IDA Pro :: Internet Explorer :: Intuit Quickbooks :: iTunes :: Keynote :: Lotus Notes :: Macromedia Freehand :: MediaWiki :: Metasploit :: Microsoft Entourage :: Microsoft Excel :: Microsoft FrontPage :: Microsoft InfoPath :: Microsoft Office :: Microsoft OneNote :: Microsoft Outlook :: Microsoft PowerPoint :: Microsoft Project :: Microsoft Publisher :: Microsoft Visio :: Microsoft Visual Studio :: Microsoft Visual Web Developer :: Microsoft Word :: Mozilla Firefox :: Mozilla Thunderbird :: MPlayer :: Mutt :: Nessus :: Netcat :: Nmap :: OllyDbg :: Outlook Express :: Pages :: PGP :: PowerShell :: QuarkXPress :: Robocopy :: rsync :: Safari :: SharePoint Server :: Wireshark :: Xcode :: Xmonad

Security:
SQL Injection :: XSS

Networking:
Border Gateway Protocol (BGP) :: Cisco IOS :: EIGRP :: First Hop Redundancy :: IEEE 802.11 :: IEEE 802.1X :: Internet Protocol (IP) :: IPsec :: IS-IS :: Multiprotocol Label Switching :: Networking :: OSPF :: Point-to-Point Protocol (PPP) :: Quality of Service (QoS) :: Spanning tree protocol (STP) :: Subnetwork :: TCP/IP :: Virtual LAN (VLAN) :: Virtual Private Network (VPN) :: WLAN

Other:
Access Control List (ACL) :: Accessibility :: Adobe Flex :: apt-get :: ASCII :: awk :: Bash :: BPMN :: Character Codes :: chmod :: Colors :: Country Codes :: Design Patterns :: DOM :: DOS :: DTD :: Exim :: ExpressionEngine :: Fonts :: Funny :: Google Analytics :: Google Voice :: Google Wave :: Incanter :: Internationalization (I18N) :: LaTeX :: Marketing :: Microformats :: mod_rewrite :: NetHack :: OAuth :: RDFa :: Regular Expressions :: Reverse Engineering :: rpm :: RSS :: Scalable Vector Graphics (SVG) :: screen :: Security :: sed :: SEO :: SSH :: SSL :: tcpdump :: TeX :: UML :: Unicode :: Usability :: Visualization :: Win32 :: WPF :: x86 :: Yahoo! Search BOSS