Good for Internet Explorer and Netscape/Mozilla based browsers.

Most are available in Firefox extensions but not all,maybe....
Interesting and educational,though.

The difference though, is that as an extension, Firefox requires more memory. A bookmarklet is executed as necessary, when clicked. As many already know, lots of extensions are the cause of memory leaks, etc. These are a good way to keep your memory footprint from ballooning to huge proportions.

I just remembered this good site too:

http://www.squarefree.com/bookmarklets/