Please list your favourite site for finding examples of well designed web elements, one per answer so they can be upvoted individually. Explain why it is good in the answer
http://patterntap.com This place has a nice, user organized, collection of various UI elements. There is quite a bit to look at, but it is a great place for ideas.
For something slightly different, here's Chris Messina's flickr collection of examples of various design patterns in the wild.
The Quince pattern library has some extensive documentation and examples: http://quince.infragistics.com/#/Main
The site itself has a unique Silverlight-driven interface, which probably warrants its own critique at some point.
I love Konigi.com for the freshness of its content, as it is updated frequently. It is also not reduced to patterns, but lives out of examples that people can vote and comment.
I find in Konigi a very approachable tool to understand how implementation of patterns can actually affect its perception and usage (mostly for such an incipient design branch as User Interface design still is)
I've always really liked Martijn van Welie's design pattern library. It's a little older than most libraries and is maintained by a single person, giving it a certain personality. Each entry includes in-depth explanation of effects, up and downsides, and examples. You can also leave comments.
I'd suggest looking at: http://www.selected-webdesign.com/
It's a huge collection of web sites, well organized in categories (style, color, design elements, etc). It's maybe not so popular as patterntap.com, but still it is a good source for inspiration.