One of the best available Open Source SIP stacks is definitively PJSIP. That project has also been one of the very first SIP stacks offering a serious ICE implementation. It's amazing to see what AG Projects has been able to realize based on PJSIP. They have written their very own SIP SIMPLE client SDK and even more: Blink!
However, a few days ago another SIP stack gathered my attention: Net::SIP, written in Perl by Steffen Ullrich. I stumbled over this library while finally writing some Perl code after being abstinent for a very long time. Sure, I'm always in touch with Perl in my daily work, but I did not use it for projects larger than small scripts anymore.
At a first sight it seems to be really well done, even if not playing in the same league as PJSIP does. I already got some keen idea on how I could misuse it for some tricky B2BUA job - at least unless OpenSIPS 2.0 is out. My first tests partially failed, as Net::SIP::Simple is not yet able to handle Record-Routing. However, that's not gonna scare me - not yet