I think a custom web browser (like make your own browser) is best since you have the greatest control. It takes more time than a toolbar, but I think it's worth the effort. The disadvantage is they work with just one rendering engine. Plugins are more difficult and they offer the least control in my opinion. A web app is the easiest to make and can work on all browsers but that means competition is much fiercer and copycats can take your idea much faster. So, I go with the custom browser. Which one is the best option for your web visitors?