Another handy function is `browse-url' which will prompt for the url but default to whatever your point is at.
If you're in a html file and you'd like to open that with your browser then `browse-url-of-file' will open it up.
Finally, and I just found this today, if you have some html code in a buffer that isn't even a file, you can open that in your browser to using `browse-url-of-buffer' and emacs will write it to a temporary file and open that in the browser for you.
