Ronja is an optical datalink 10Mbps full duplex over 1.4km:
Community networks are often having problems with wireless device interference and aggregation of unreliability on many-hop paths. Community network participants often solve these problems by installing Ronja.
Highest concentration of Ronja devices in community network in a single city can be observed in Prague, CZ. The community network is CZFree.Net. They are using it mainly for upgrading existing WiFi backbone (AP to AP) links - to get rid of packetloss, roundtrip and hangup problems.
You can view list of all the links in this community network with aerial maps and photogaleries.
There are other community networks using Ronja, like hkfree.org.