I just posted something to del.icio.us with my favorite client, Pukka, and up pops a notification that it's been revved to 1.4. Now as much as I like Pukka (above all for its multiple account functionality) that would probably not be worth a post, except for the fact that version 1.4 adds the one thing I've always most wanted: duplicate warning! The whole point (as far as I'm concerned) of things like del.icio.us is that I don't have to remember what I'm posting there, but by the same token that means I often post the same thing several times. The new version of Pukka "knows" everything you've posted, and if you try to post a duplicate it will tell you: you're "given the original post date along with the option to cancel, continue editing as new and overwriting the old post, or populating Pukka's editing form with the old post for review or refinement." In addition, 1.4 adds hierarchical menu access to all your bookmarks (through their tags) in the Dock icon, as well as a few other things as detailed on the Pukka blog. A hearty TUAW 'attaboy for Justin Miller over at Code Sorcery Workshop for this one.
Pukka is a very reasonable $5 and a 14 day demo is available. If you use del.icio.us with a Mac you should definitely get Pukka.
[Update: added note about the hierarchical menu in the Dock icon]