StumbleUpon is a pretty unique site as far as social browsing sites go; no site with a similar concept (e.i. one click ‘random’ surfing through sites recommended by the community) has reached anywhere near its level of popularity. However, unlike - for example - Digg or Delicious, it requires users to do one extra step: install a browser plugin, which is something many users look down upon.
Not any longer. StumbleUpon now offers a web toolbar, which is basically a (relatively) unobtrusive frame at the top of the webpage, with the full functionality of the browser plugin. To use it, just go to the StumbleUpon homepage and click on a link. You can stumble sites (category stumbling is also available), rate them, send them to friends and read comments - all without installing anything. Of course, to get the most out of StumbleUpon you should register with the site, which - if you ask me - is pretty much the same thing as installing the toolbar. But, one extra step is removed and it can be a world of difference.