Allegedly, wikimatrix can help you pick a wiki provider - I saw references elsewhere to wikidot coming out on top of the free ones, but the discussions where that was said are several years old - ymmv.

You say free - if you already have a web hosting account, a number of the really good wiki packages are free & open source. I run a couple of mediawiki instances on mine, and it's not hard to maintain. My host costs me 40-50 dollars a year, but if all i wanted to do was a modest wiki, I could probably get hosting for around $20 a year.