What is the point in paying a software when good alternatives are around for free ? That is the true case today with a lot of applications. For Photoshop we have GIMP, for Illustrator, we have Inkscape, for Windows, we have Ubuntu. So goes the list of free and open source alternatives.
It is true that free forum software has taken a back seat recently. They have some features missing which people would have wanted. But if you have the guts to come out of the PHP realm and try some free software, Discourse might be the name you want to talk about. Discourse is sculptured using Ruby on Rails and uses PostgreSQL for the database. It is a very feature rich forum software with a vision beyond time. They are ready for the upcoming challenges and thinks about the future. This community contributed software has the ability to match most feature available in leading paid forum software. I would bet that some featured Discourse has might even be missing in leading proprietary software.
A lot of people have started to lean on Discourse when they start to think about having their own community. Be it for customer support, product loyalists, personalized support, Discourse has made its place.