StumbleUpon is different from the online bookmarking nich -- StumbleUpon is essentially a websurfing toolbar used to find new websites based on what you and others "like" you like. Recognize any aliases?
- spurl.net is the online manager I've used the most historically. It has been a bit slow the last while and has a few shortcomings that make using it for large bookmark collections a bit tedius. The eventually-coming new version is supposed to be pretty awesome, though.
- linkaGoGo is the service I've been using primarily lately. While its interface is a bit cluttered and not necessarily pleasant to look at, linkaGoGo is unmatched in terms of features -- and it scales splendidly with larger bookmark collections. Each bookmark can have an associated image, rss feed and "smart bookmark" -- the latter works like opera/firefox "keyword" searches. Add linkaGoGo search to your searchbar and you can type in the alias of your bookmark to go to the bookmark or type the alias and some keywords and you can use that bookmark's associated search url -- powerful, handy stuff.
- del.icio.us is probably the best known online bookmark manager. It was the first that I know of to effectively use tags and its social aspects work but I haven't used it that much. Spurl can add links to del.icio.us, so I have a del.icio.us account that is a mirror of my spurl account.
- ma.gnolia.com is pretty, nice elegant aesthetic but is new and seems rather limited in features. Its emphasis appears to be on social bookmarks/sharing.
- shadows is a community driven bookmarking service.
- wink is another community driven bookmarking service.
Those are the ones, off the top of my head, that I've opened accounts with. Obviously only have input on a few of them -- generally, I don't see the difference among the social driven ones and don't spend enough time with any of them to detect their subtle differences.
What I have observed is that there are generally two classes of URL archival sites -- bookmark services and social bookmark services. The bookmark services tend to have a strong features for personal bookmarking -- including bulk operations, folder-based organization (with tags as a secondary method usually), and link analysis. These days, they also include a social aspect or some form of link recommendation based on user consensus, but these social aspects are afterthoughts.
The social bookmark services, on the other hand, have only basic bookmark feature sets -- your general url/name/comment information, and tag based organization. They generally have a much more fleshed out and feature-rich social aspect, though -- since that's the driving force of their service.
As a rule, the personal bookmark services are you building your own search engine/bookmarks -- while the social bookmarking services are about a community building a search engine -- a micro-web of sorts. One is more equipped to organize you; the other more equipped to help you find new things.