Shortwave by Shaun Inman is one of the most powerful bookmarklets that I use. Clicking on Shortwave in your bookmark bar simply opens a JavaScript dialog box that asks for a command. For example entering:

g Shortwave

will load the current page with the Google search results for ‘shortwave.’ And entering the command:

ip

Will return your current IP address, but the most powerful/interesting thing about Shortwave, is the ability to define your own ‘waves.’ Shaun gives you a default set of these waves in this text file: http://shortwaveapp.com/waves.txt, but you can modify it, and upload it to a server somewhere, and use your own list of waves, rather than using his.

It’s really pretty genius, and extremely simple to configure and use. Give it a shot, and let me know how you use Shortwave in the comments.