a. What kind of functionality are you trying to expose? b. End user constraints, much like bostonbiz raised

Tumblr has a bookmarklet, but its limited because it does not let you tag your tumblrs. I find that limiting, but I guess thats what they have settled for. On the other hand, delicious offers a toolbar with a buttload of functionality built in.

This may sound silly, but I guess you could start off with a bookmarklet, and let the users decide. Just a thought...