Well, the markets have worked their magic (this one time, at least), as stateside competitor MOG this morning introduced its own free plan.
Since the two company's catalogues are roughly equivalent (around 15 millon tracks each, instantly available on demand), I'm thinking MOG may be the better deal. Its playlist offerings are much better, and when you share links to songs with friends, they don't have to be subscribers in order to listen.
Of course, with both companies, you'll still have to pay $10 a month if you want to listen on your mobile devices. But if computers are your streaming media device of choice, the free plans are hard to beat.
Plus, the savings will offset the recent doubling of your Netflix bill.