Unfortunately I can't remember where all these came from, so I'm just going to paste the code in here for most of them. I've also left out a lot of bookmarklets whose functions are available in the Web Developer extension.
- Bloglines Citations - find links from other pages
- Tasty - show tags on del.icio.us [link]
- Waypath It! - find similar pages [link]
- Playr - play linked media files [link]
- Mouseover DOM Inspector (external link) - show DOM attributes for parts of the current page
- Remember Password - allows browsers to remember passwords for forms with autocomplete="off" [link]
- Cite - quote selected text with attributes
- Colour Generated Source - show the source code for the current page [link]
- Translate - send the current page through Google's translator
- TinyURL - create a short URL
- Webjay - add a media file to a Webjay playlist
- AllAbstracts - send PubMed links to HubMed
See also Jesse Ruderman's large collection of bookmarklets.