Trailfire allows you to create a list of links, and also annotate the actual webpages in the list, similar to diigo, and also ButterFly, perhaps even clipmarks.
Not only that, from the annotation on the actual webpage you can also actually link to another webpage that is on the same list…this adds more the “web” element to the term “social web”.
But it’s more than just a gate way to links, each link (mark) in the list (trail) is viewed within Trailfire…since you view the actual webpages within Trailfire it makes it more of a portal.
Here are some use cases for trails.
NOTE: del.icio.us can only create a list if you consider a tag as a list heading, whereas most list services allow you to create a title for a list of links, and also to describe it with tags.
In saying this Trailfire doesn’t allow you to tag your Trails or marks, but there is a keyword search cloud, the results display the marks containing this keyword, and each “mark” displays a link to the trail it belongs to, see a search for the term “Google“.
Read about it here.