Freifunk is a community which promotes open-access mesh network, mostly based on wireless links. Strictly speaking, Freifunk is only one flavour or some type of brand name for that concept, and those communities refer to themselves as Freifunk mainly in german-speaking localities as the name Freifunk derives from the two german words "frei" (means free) and "funk" (means radio).
What's common to most local Freifunk communities is the widespread use of OLSR as routing protocol, which is especially designed for the task of routing packets in such volatile environments. There are a couple larger setups, with Berlin being probably the world-largest testbed (reportedly between 500 and 1000 nodes on-air) for OLSR as routing daemon. Due to the fact of limited coverage of those wireless networks, the interconnection of wireless networks by means of VPNs through the Internet (usually using tinc) is widespread.
OLSR in itself is already capable of routing IPv6, still it hasn't seen much usage yet along with IPv6.