They all use the same forum hosting script (simplemachineforums).
I believe the actual maker of the SMF script is
HereSMF For Free just allow free hosting of the script on their servers, in exchange with google adverts being placed on the forum.
Its a great service and ad removal is ALOT cheaper than buying forum scripts
Vbulletin and
IPB cost alot of money and require some experience to get up and running.
Then you have to add hosting costs ontop of any forum script costs!
SMF For Free is good for most new webmasters.
Feel free to pm me for a great FREE alternative to SMF For Free (Vbulletin and IPB hosting)