I believe that most other implementations are unsuitable for displacing OpenSSL for licensing reasons alone. I didn't look at every other implementation since I was looking for one that supported a specific mode, but the majority of them are GPL with the option to license for proprietary code. GnuTLS is LGPL, at least.

Wikipedia has a pretty good list of possible implementations:

http://en.wikipedia.org/wiki/Comparison_of_TLS_Implementatio...