http://www.socialgo.com/ Basic is free, advertising driven. Limited functionality, but appears to have no less than Ning.
http://www.zimbio.com Basic, but seems to do the trick
http://www.grouply.com/ Offering themselves as a Ning alternative. Options to migrate users across
http://bricabox.com/ Not tried this, so limited knowledge of it
http://www.bigtent.com/ This looks easy and straightforward to create a group
http://www.spruz.com/ Also setting itself up as a Ning migration site. Good collection of features.
http://www.snappville.com/ This looks like it's free, but I'm not sure, and not convinced by their patter, either.
http://www.crowdvine.com/home Freebie version, limited functionality
http://www.mixxt.com/ Looks like it does the job, but doesn't say much about itself
http://www.webs.com/ has a freebie version as well as paid
http://grou.ps/ This looks quite funky, but appears to be a commercial product.
I think it's a really shabby move by Ning, who have always made it clear that they have free resources. If you check out their site in the Internet Archive you'll see that they often make the statement "Ning is the only online service where you can create, customize, and share your own Social Network for free in seconds." Even going through the terms and conditions there's very little that refers to the free/commercial element of the site, and no-where does it even seem to imply that they're going to, or may possibly, revoke the free element; there's just the catchall 'we can change our minds on any and everything' clause, which everyone has.
'Free' is an interesting concept, and one that lots of social media organisations use. It's certainly one of the questions that I get asked on my courses - 'how do people make money out of these services', and the answer is always 'advertising and loss leader free limited accounts'. Ning was using advertising, but apparently that's not enough for them, hence their move now. They're entitled to do it of course, but is it sane and sensible? I'd say not, because although they may have not, to the letter of the law, lied to their users I don't think they're going to be in a position to claim any high ground - ever again. How about their users - are they going to be happy paying for a service from a company that they can't really trust? If I had a Ning Network (which I don't - I never particularly liked the offering, so was simply a member of a couple of groups) there is NO way that I'd give them money - not because the service wasn't any good, but because they're not to be trusted.
I'm reminded of the situation with Pageflakes - the home/start page company. They used to have a really nice product, which was then sold to a company that wanted to make money. I can fully understand this, and if they'd asked their users for ideas I'd certainly have been happy to have moved to a subscription based model. However, they didn't - they simply forced advertising (and some of it very dubious, certainly not appropriate for schools) on users, in a box on every page. Consequently people left in droves, Netvibes flourished (and is now in profit), and Pageflakes is now a very poor relation.
The money aspect isn't actually (surprisingly) that important in many respects - it's the approach to the user base. What people will remember about Ning in the long run is not that they went from a free to a paid model, it's that they betrayed their user base. My personal prediction is that Ning isn't going to last for much longer - I give it about a year.