In Britain, a free house, (as opposed to a tied house), is a pub that isn't 'tied' to a particular brewery. The terms came into use around the time of the Industrial Revolution.
Being a 'free house' means, in theory at least, is that the landlord can buy and sell beer from any brewery he choses. In the days of yore, being a 'free house' was a great marketing advantage as ale drinkers would gravitate towards pubs offering a selection of beers, (often with 'guest' beers too).
Nowadays, despite CAMRA's best efforts 'real ale' consumption has declined latterly as the younger drinker has turned towards industrialised psuedo-lager and the dreaded alcopop. A 'free house' may simply be a pub that isn't owned by one of the two brewery conglomerates that dominate the British pub trade. Probably still worth checking out, though!
Cheers!
References
http://www.cam.net.uk/camra/