Bookmarklets are, IMHO, being used for more and more funky functions.
I really like the Facebook and Tumblr bookmarklets that scoop out information about the type of content you want to post and then sends them off to the respective sites.
John Gruber recently reposted his JavaScript Bookmarklet Builder - which is useful, but if you're planning on writing a bookmarklet, there's a lot more to consider than just escaping characters.
So prompted by this opinion, I've compiled a list of tips I've learnt from writing various bookmarklets, from simple jQuery injection to the Speech Bubble app.
Please have a read if you're interested in Bookmarklet Coding.