I found it quite ironic that we can view and control our cars from our phones and tablets but not from our computers, but I share the concerns with sending my credentials over a third-party site, so I have written a bookmarklet which dresses up the Tesla portal (documented by timdorr) to allow login and control directly from there. That way, all the communications is with Tesla's server directly.
Since this new thread was created, I figured that I would post a link to my posting about my web app to control your car (the third-party site alluded to above): Model S REST API - Page 22