I've been playing with different softphones a bit lately. Anything I've tried seems to work. Here are some of my favorites so far:

Windows:
  • 3CX - easy to set up and use, same look and feel as Android is a plus in my mind
  • X-Lite


Mac OSX:
I'm just starting to use the Mac clients, so I don't have much to report on them yet.


Android:
  • CSipSimple - noticed some issues with voice quality of receiving end, may be WiFi related, though
  • 3CX - had some trouble with sending DTMF tones to VOIPo voicemail (the main reason I use a soft phone on my cell phone). If not for that, this would probably be my preferred Android softphone.


--Jason