Flock

Flock is a new type of browser that has some interesting features. As well as using del.icio.us as its bookmark source (and the option to tag favourites), it incorporates a built-in content manager (Clucene) that indexes the content of all the pages you visit. This then allows for instant search and tagging within your history, removing the need to remember 'which page had what on it'.

Flock also has a few other handy features, such as built-in blogging and RSS/Atom aggregation.