It's actually fairly simple and free. Tapatalk offers this service if a community wants to join into it. Huge forums such as Anandtech use it. Just go up to Activate Tapatalk, select vBulletin (which is what this community uses) and do some quick integration steps. Of course this has to be done with Site Admins. Most car forums I visit have a Tapatalk integration for those with mobile devices.