deliGoo is a nifty new add on for Firefox 2.0+ and IE 6.0+ that mashes up del.icio.us with Google Custom Search. The way it works is by creating a Google Custom Search engine based on all of a user's del.icio.us bookmarks, all of the bookmarks under a single tag, or all of the bookmarks under a single tag from a single user.
This can be very helpful for people who want to find something from among many untagged bookmarks, or for people who don't care to bother with tags but still want to get some utility out of del.icio.us. deliGoo can also be helpful to search among the collective knowledge of a large group of people (i.e., if I search only among sites tagged with "MySQL," I am presumably searching among the knowledge of a bunch of database gurus).
Unfortunately, deliGoo has some oddities and drawbacks that make it less useful than it could be. One of the major things that jumped out at me is just how useless the Firefox extension actually is. The purpose of the extension is to provide quick access to a deliGoo search engine, however, all it really does is provide a link to their search page. A simple bookmark would work just as well.
The biggest drawback, however, is that creating a search engine takes time, and it has to recreate yours every time you use it. (At least, every time you navigate back to the page ... if you create a search engine and keep the results page open, it will save that search engine for as long as the page is active.) In the interest of time, deliGoo was set up so that you could use it without registering, but it would make a lot more sense if it remembered you and didn't have to rebuild your engine every time you needed to search.
I think the service would be much more useful if it let users sign up and create one or more custom search engines and had them readily accessible without having to wait for Google to index the sites each time. The site could add a manual or scheduled update option, so that if it has been a week and you've added 100 new bookmarks to your del.icio.us account, you can update your engine -- but on your schedule so you aren't forced to wait when you want access to your information quickly.