Bookmarklets
A bookmarklet is a bookmark which has got some code like JavaScript (most common) embedded in it. So, instead of there being a URL like "http://whatever.com" in the bookmark like usual, there is JavaScript instead like "javascript:alert('hello');". When you click the bookmarklet bookmark, this code is executed on top of any existing web page, so it's a way of adding code to a web page.
deBuglet
The deBuglet bookmarklet below can be useful for debugging scripts and creates two windows on top of whatever web page you are on: an input window that you enter JavaScript code into, and an output window for displaying any, well, output by using the built-in utility functions. Entering "deBuglet.help();" will display these functions.
Drag the image to the bookmarks toolbar to create a deBuglet bookmarklet:
deBugletOr click on it to try it without creating a bookmark.
icOnify
The icOnify bookmarklet will change the icon (favicon) on the tab of the site you are on which will allow you to assign a favicon to a blank bookmark. Favicons are useful in bookmarks because you can delete the name and thus save space on the bookmarks toolbar.
It will only persist if the page doesn't already have a favicon. If it does, refreshing the page will wipe out the icOnify one.
Drag the image to the bookmarks toolbar to create an icOnify bookmarklet:
icOnifyOr click on it to try it without creating a bookmark, although there is no point changing the favicon on this page because it already has one.
To put the favicon in a (blank) bookmark, the procedure is: 1) Create bookmark (drag the tab to the bookmarks toolbar), 2) Use icOnify to change the favicon on the tab. Bookmark should change on the bookmark. Unfortunately, when you read the page with the icOnified bookmark, the page's tab will be blank again. But the bookmark should keep the favicon, which is the main objective.
Seems to work with .ico, .png, .bmp, .jpg and .gif files. TIFFs don't seem to work on Firefox.
(Haven't figured out how to bind an icon to the bookmarklets themselves.)