Other than my new site (which I am working very hard on, and really think will be great) the web service that I'd like to see most in 2006, is a bookmark tracking aggregator.
Every few days I check the number of times that Lopico has been bookmarked on del.icio.us, and sometimes Furl, but there are also a handful of other bookmarking sites such as yahoo myweb 2.0, de.lirio.us, clipmarks, and simpy. Del.icio.us is the most important for now, but to truly use bookmarking as a tracking tool I'd like to be able to see data from all of the sites compiled onto one page. Sort of like clusty meets social bookmarking.
Will it happen? Who knows, but if it does I'm sure I won't be the only one using it.