Also a big fan of Mint. They were the first MVNO, and perhaps even carrier, that I can remember having a really slick website, too. Seemed very modern and forward looking. Their founder, Rizwan, is also around on the subreddit and will generally answer valid complaints - which is pretty unheard of for a company of that size.

Also, Ryan Reynolds sent out a postcard once, and also a voicemail, if that's your thing.