The JavaScript libraries that I got these code snippets from, and other useful sites:

• http://www.dynamicdrive.com/ - recommended
• http://www.hotscripts.com/JavaScript/ - recommended
• http://www.brothercake.com/scripts/ - not many scripts available, but still recommended
• http://www.scriptsearch.com/JavaScript/Scripts/
• http://javascript.internet.com/ - easy to use, but often broken in modern browsers or not Y2K compliant
• http://www.designerwiz.com/javascript_cut_paste_code_list.htm - professional stuff, but difficult to navigate site due to barrage of marketing
• http://www.mattkruse.com/javascript/ - mix of complete scripts and widgets to include in other scripts
• http://www.quirksmode.org/index.html?/js/contents.html - recommended although more theoretical, tries to teach you JavaScript
• http://evolt.jeffhowden.com/jeff/code/ - for experienced developers