BGP - existing implementations

Zebra: GPL, makes heavy use of cooperative threads. Suffers from losing sessions while busy. Documentation and error messages in japanese or missing. Commercialized about 3 years ago, thus mostly dead.
Quagga: frustrated zebra users try to fix the worst bugs
Cisco: proprietary, only comes bundled with overpriced hardware. Usually works ok, unless you happen to hit one of its countless bugs, or the tiny CPUs they use are swamped with work.
Juniper's JunOS: apparently works ok, but not free either.