OpenBGPD

Existing Implementations
Zebra: GPL, makes heavy use of cooperative threads. Suffers from losing sessions while busy. Commercialized long time ago, thus mostly dead.
Quagga: frustrated zebra users try to fix the worst bugs
xorp: university project far from production ready
Cisco: proprietary, only works on their overpriced but underpowered routers.
Other comercial solutions like Juniper or Foundry

At the time OpenBGPD was started no high quality open-source project was available.