The Bookmarks Widget for Plasma is now part of the kdeplasma-addons module, so it should be included in the KDE SC 4.5 (coming in summer 2010). It a pretty simple widget for now (thus version 0.1) which offers you quick access to your (Konqueror) bookmarks, using the plain-old-style menu-with-submenu, as can be seen here (yes, KDE is not a project, but a community, too lazy to update the example data):
You can also set another bookmarks submenu as base folder. Or start the bookmarks editor from the context menu.
More plasmoid UI alternatives, like a Kickoff-style menu navigation, yet to come.
Also hoping for a soonish movement of the bookmark system to Akonadi and Nepomuk, so integration of other bookmarks systems (delicious, Firefox, Chrome, whatever) is done at the right level. It won’t be done by this widget (code). Anybody interested to join this work?