I want to have my custom field use HTML, how do I do that in such a way that I can use a rich text editor to view the content?
You can use Advanced Custom Fields plugin.
It's pretty easy to use and very powerful, so it's the best choice right now, I guess.
I sometimes use Custom fields Template : http://wordpress.org/extend/plugins/custom-field-template/
You can attribute a type to your field (wysiwyg, list, image...)
Found this, but I can't comment on whether the approach used is good or not, but it certainly covers what you want to do and more (storing and retrieving data that is not stored as post metadata.)