For all my fellow Mac users, I also wanted to mention that the simplicity of the "Google Telephone" app makes it a pretty stellar softphone application:

http://www.tlphn.com/

http://itunes.apple.com/us/app/telep...06825478?mt=12

Google Telephone has fewer features that X-Lite, of course, but its simplicity and ease of use gets a lot of points in my book.