Bookmarklets are simple one-line JavaScripts, implemented as ordinary bookmarks. You can use bookmarklets to add macros and additional features to your browser. Bookmarklets are cool because:
(The above is taken from http://www.webreference.com/js/column35/)
You find a number of handy bookmarklets at http://xdeb.org/bookmarklets. MailTo for mailing a weblink, post to del.icio.us, search Versiontracker etc.
The only reason I don't post them here directly are that links with embedded JavaScripts are not supported in Tavi, for security reasons I guss so I don't want to hack it.
Good resources are: