This is a work in progress, motivated by self-interest, because I wanted to have a handy list of pattern libraries someplace where I'd remember to look.

Web/UI

• Search Patterns (Peter Morville)
• Yahoo Design Pattern Library (Yahoo)
• Interaction Design Pattern Library (Welie) 
• UI Patterns (Anders Toxboe) 
• Web Patterns (UC Berkeley) Now requires a password. Boo! 
• UI Pattern Factory (Lammi & Hocksell)
• Design Patterns - Flickr set (Chris Messina)
• Footer Patterns - Flickr set (Mark Pollard)
• Information Design Patterns (Christian Behrens)
• Sign-up Forms (Smashing Magazine)
• Quince: UX Patterns Explorer (Silverlight app from Infragistics)
• Designing Web Interfaces (Bill Scott & Theresa Neil)
• 12 Standard Screen Patterns (Theresa Neil)
• E-Commerce Patterns (Ecommr)
• Designing Social Interfaces (Crumlish & Malone)
• Pattern Browser (University of Applied Sciences, Potsdam)
• Sun Web App Guidelines
• jQuery UI patterns (with code)
• Designing Interfaces (Jennifer Tidwell)
• Pattern Tap

Mobile

• Mobile UI Pattern Library (Little Springs Design)
• Forum Nokia

Wireframes

• Wireframes Magazine
• Eightshapes Unify System

How-to

• The Elements of a Design Pattern (UIE/Jared Spool)
• Design Patterns: Part 1 (Luke Wroblewski article series)

Other pattern library lists

• The Interaction Design Patterns Page (Tom Erickson)
• Design Patterns (Dey Alexander Consulting)
• 40+ Helpful Resources on UI Design Patterns (Smashing Magazine)

Patterns in related disciplines

• Patterns for object oriented design

[ If you know any more libraries or resources I should include, please let me know. -- Kim ]