Building a toolbar

Hi,

I would like to create a toolbar that will have access to the html code of the current page and according to the content will be able to add stuff to the browsing experience. A good example is a spell-check tool that can read the content and add buttons to the page.

I have a lot of programing experience but I don't know where exactly to start.

Hope someone can help..
Thanks

Start by picking which browser you want to build this for. For Chrome and FireFox you could build a greasemonkey script, which is the absolute easiest way to do something like that. You can also write native extensions for either, although not as easily and not cross browser.