Cocoalicious is a kick-ass client application for MacOSX that connects to your del.icio.us account, making it easy/fast to search, list, add, index, etc. bookmarks in del.icio.us.
It provides some cool features, like adding star ratings to bookmarks, and has a built-in webkit page displayer. Very handy. It's using the back-end API for del.icio.us, and feels much snappier than the bookmarklet version (or even the Firefox extension).
And it has the ability to make fulltext indexes (indices?) of all of the bookmarked pages, to make searching even more powerful. I've set it to go through and index each of my 800+ bookmarks, and it's been chewing on that for a few minutes now. I'll try some searches based on that index when it's done.
The bookmark editor pane has tag autocompletion, so you don't have to think too much about what tags you've used previously. It will provide you with a list of tags that match whatever letters you've started typing.
The only thing I can think of that would make Cocoalicious even better would be the addition of a "Tag" button to Safari's address bar, so I don't have to Command+Tab over to Cocoalicious, and hit Command+Shift+N to create a new bookmark of whatever is frontmost in Safari.
Update:
I just found a handy dashboard widget called DashLicious that does much of what Cocoalicious does - autocompletion of tags, pulling the URL of the frontmost Safari (or NetNewsWire) page.
It's a one-way trip, so it doesn't provide a way to list your bookmarks. But, it does offer a very quick way of posting new bookmarks - never more than an F12 keystroke away...
DashLicious isn't new - it was posted to the Apple Dashboard Widgets site in June 2005 - I just haven't been giving much love to Dashboard lately because it's another things that seems to suck up resources just for something to do...
Update 2: The changelog on the Cocoalicious project page includes a reference to a bookmarklet that will send the current page to Cocoalicious for tagging into del.icio.us. Works like a charm! Maybe there doesn't need to be a Safari button added after all... What's even cooler, all I have to do is hit Command+6 to trigger the sixth non-folder item in my Safari bookmarks bar, which corresponds to the Cocoalicious bookmarklet. So a quick key sequence sends the page to Cocoalicious for tagging. Very slick.
Update 3: I just realized that Cocoalicious is missing something that I really find useful in the del.icio.us web interface - the website displays how many other people are sharing a bookmark, with a link to view all of those people. Great for finding similar links, and sorting bookmarks by "importance". I have no idea if the del.icio.us API supports that kind of information, but it would sure be cool if it did...