You may or may not have come across bookmarklets, but they are very useful things.  Essentially, it is a bookmark that has added functionality.  They do a similar job to simple browser plug-ins (add-ons, extensions, call them what you will) without the need to install anything.

Take a look at this page – it lists some very cool bookmarklets, some of which I knew and a few that I hadn’t come across.  Very useful to have them all on one page, though.