Here is a very useful list oif user Experience (UX) Design Patterns that I came across in a linkedin discussion. Feel free to comment and add any more Design Pattern Repositories that have been missed.

http://ui-patterns.com/

http://uipatternfactory.com/

http://quince.infragistics.com/UX-Design-Patterns.aspx

http://developer.yahoo.com/ypatterns/

http://www.welie.com/patterns/

http://www.designingsocialinterfaces.com/patterns.wiki/index.php?title=Main_Page

Updated ~8/15/09

More Pattern libraries. - Thanks to http://designingwebinterfaces.com/

• Designing Interfaces - Jenifer Tidwell
• The Design of Sites Patterns
  
• blink design library
• User Interface Design Patterns: Introduction
• Designing Social Interfaces
  
• WebPatterns : News from the WebPatterns organization
• Wikipatterns - Wiki Patterns
• Designing Gestural Interfaces
• Game Interaction Design Pattern Library
• Flickr Collection: Design Patterns - Factory Joe
• UI Pattern Factory
• The Pattern Gallery
• 400 So Far
• findability.org: Search Patterns
• Interaction Design Patterns — maintained by Tom Erickson
• Smiley Cat’s Elements of Design
• KONIGI’s Latest Interfaces

Updated 8/23/09

http://extjs.com/examples/#overview
is a great resource also, especially for RIA patterns.
Blink Interactive has some great posts also with various patterns
- http://designlibrary.blinkinteractive.com/
Loren Baxter has Axure design patterns on his blog
- http://www.acleandesign.com/2008/09/axure-design-pattern-library-v2/

 PeakUsability Have just discovered http://uitrends.com/. Looks like a great pattern library of trends in UI design as it develops over time.

Updated 9/28/09

IBM - I just came across 
https://www-01.ibm.com/software/ucd/designpatterns.html at IBM but they only have 

* Auto-complete 
* Details on Demand 
* Dynamic Forms (New!)