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.
According to CNET, the folks at StumbleUpon have been mulling the idea to ditch the browser plugin over for years, but they were afraid that users would simply click that stumble button without rating the sites and participating in the community. However, clicking pretty much anything on the Web toolbar without being logged in will remind you that you should register to fully enjoy StumbleUpon, which will probably be enough to persuade frequent users to do so.
Together with the Web toolbar, StumbleUpon is launching a partner program, enabling sites to offer a "Stumble This" button with a counter representing the number of votes for your site. The more people click on it, the more popular your site is within StumbleUpon, and the more it'll show up to their users while they're stumbling. Furthermore, you can use the button to jump to related content, a functionality offered by many other services, plugins and widgets, but a useful one nevertheless.