This may be old hat to many, but I thought it may be useful for someone, somewhere. Here's how I do it using Firefox.
Step one: Register with del.icio.us
Create an account at del.icio.us. The bookmarks you create here will be viewable by anyone (mine is linked at the side of this page, under my profile), but you need the password to add anything. Click on the Register link.
Step two: Add a couple of important bookmarks
The first thing you see when you register is the "About" page, which has some important links you want to save. Drag the "popup post to del.icio.us" link to your links bar (the part of your browser just below the address area). I'd also suggest using the "my del.icio.us" link as your home page.
Step three: Save your existing bookmarks
Copy your existing favorites/bookmarks to del.icio.us by going to each page and clicking on the popup post that you put in your links bar. Type at least one tag (or keyword) for each bookmark. Don't worry, you can edit these later if you want to organize your bookmarks in a different way. I'd suggest just starting with one tag for each bookmark -- a simple one, like "news" or "travel".
Step four: Subscribe to your bookmarks
When you're finished, your page at del.icio.us will have all of your bookmarks organized into categories, using the keywords (tags) that you assigned them. They're listed on the righthand side of the page, with the number of bookmarks in each category. Now use Firefox's ability (since 1.0PR) to subscribe to some of those categories using RSS (Really Simple Syndication).
For example, if you've saved several bookmarks under "news," click news and then click on the red RSS button at the bottom right of the browser, then choose the "Subscribe" option. You'll get a dialog box like so:
Rename the bookmark (in this example, "news" would be a good choice) and click on OK.
After you've done that for a few catagories, your bookmarks will have several folders, all containing a list of the links stored on del.icio.us.
Now, if I'm at work and I spot a new news site I want to save, I simply add it and it's available everywhere, as long as I've subscribed to that category.