I've been using the del.icio.us extension for Firefox for a few months now. My demands for an online bookmark system were simple: it had to be available from anywhere, even if I'm at someone else's machine (easy), and it also had to integrate seamlessly with the existing browser bookmark interface (erk, not so easy).
Well, heck, the del.icio.us extension does both admirably. I can continue to use the bookmark treeview I'm used to, or use a search-as-you-type approach, or browse by tag. To tell you the truth, I'm hooked on the search-as-you-type so completely now I almost never browse my bookmarks by folder. I have a *lot* of bookmarks now anyway, and trying to organize or navigate them by digging down through nested folders is a fool's errand.
Adding a new bookmark is also very quick indeed. I don't have to choose a folder it belongs in, all I have to do is label it with some tags. Tags are recommended to me from popular tags other people have used on the same page, plus tags I've used before. I'm also given complete-as-you-type suggestions with the number of times I've used the tag before. Brilliant.
del.icio.us Bookmarks Firefox add-on: https://addons.mozilla.org/en-US/firefox/addon/3615