Update: Now there are multiple bookmarklets here.
It works in the usual way. Add it to your bookmarks then click it on any page. If you select some text on the page before you click it, that text will also be copied into the compose screen for you.
With this change, the compose screen supports 4 new url arguments:
s = text selection,
t = page title, and
u = page url, on the new target url
For example, this url will load a compose screen like:
I'm the awesome body
This is the text I selected
Title of Awesome http://awesome.tld/
If you do roll your own, please let us know in the comments below. 🙂