This would simply be a del.icio.us-like or blogmarks-like (choose the one you want) application BUT where you could host it on your own website.

First reason of self-hosting, not being "filed" by a company. You're also not sure it won't end its service and you'll lose your bookmarks.

The third reason is because of a feature I thought : imagine you install it on a company intranet. Every employee could put links related to his work so that first he could find it later and secondly it could benefit to other employees and/or another employee that would have to continue the work of the first one ...
So the employee finds a new link, he adds it to the portal and tags it correctly with keywords about what is in this link of course, but he could also add a tag giving a target (e.g. if it's interesting for every employee in the company or every employee in his service or just for people working on a specific project, ...).

On a more personal-use point of view, I would also see a public/private system so that you can publicly share links with people coming on your website (and why not a corresponding RSS feed ?) but also keep some personal bookmarks (like the one to this photo album of your nephew for instance :) ).
I would also add a "temp" system for keeping bookmarks that you know might be interesting but you don't have time to read (I said "system" and not "tag" as Nicolas is against the temp tag ;) ).

Well, that's more or less all I had to say about it, maybe I'll do it in a coming future (maybe based on hariCow ? ;) )

PS: the date you entered the bookmark would also be kept to find back bookmarks by date