People often look at companies that create browser toolbars (such as Others Online), scratch their head, and say "jeez, getting someone to download a toolbar is kinda hard, isn't it?". Yes, in fact it is, but so is getting people to come to your Web site repeatedly. There are two very important things to remember about toolbars.
First, toolbars that provide value ACROSS a user's browsing experience are much more likely to be downloaded than toolbars that simply emulate a Web site experience. Take search bars -- I search the Web all the time and I don't have a Google, Yahoo or MSN toolbar. Why should I? If I don't have Google open in a browser window already, then it's never more than 1 click away. In contrast, tools for journalists and researchers (who bookmark, annotate, etc. as they comb the Web) are much more appropriately implemented as toolbars because they add value while users browse the Web, on any/every page. If you have a truly unique user experience with value provided across a user's browsing experience, then people WILL download a toolbar.
Second, you need fewer toolbar users to "make" the business than you need Web site visitors. Consider the use patterns of a Web site versus a toolbar, and think in terms of pageviews. If you have a Web site, then you know it's extremely difficult to get users to the site, and then come back again. Even a very popular site like Zillow might only average 2-3 visits per month from each unique visitor.
Let's say you had 1M unique users visit your Web site in a month, with each user visiting 3x in that month, and generating 5 pageviews. That's 15M pageviews. Now let's say you had 100K toolbar users, with 75% of them online each day. Online users generally average about 120 pageviews per day (across all sites) so that would be 27M pageviews. So roughly speaking, with 1/10th the number of toolbar users you generate almost 2x the number of pageviews.
Of course, toolbars couldn't possibly get away with showing an ad on every one of those pageviews, but as long as the product utility justifies it, the toolbar CAN utilize that pageview to generate accretive user value. (Profiling, for instance, in which you can make your ad 10x more relevant to the user and also 10x more profitable.)