We use an Asterisk based solution as our PBX, and although we've found hardphones work flawlessly, we have yet to find a reliable softphone.
The basic requirement is that it allows multiple calls and transfers.
I don't have a comprehensive list, but most of what we've tried adds delay to the voice (echo), garbles the sound, doesn't allow more than one call at a time, or just plain won't work.
On Windows we're fine (SJPhone), and although I'd prefer to use the same softphone for both OSes, I will settle for a good Linux-only one.
Any recommendations?
I have looked at quite a few of the soft phone clients one of my favorites is x-lite http://www.counterpath.net/x-lite-download.html I don't like that you can't transfer (with the free version) but it seems to be a very solid client. You should also check out this comparison http://en.wikipedia.org/wiki/Comparison_of_VoIP_software
I have had good success using ZoIPer (formerly idefisk) under both Linux and Windows.
One major advantage of that client is that it supports both IAX and SIP. The free version supports two line registration profiles and they have paid versions as well.
The only one I know of is Ekiga. Never used it so can't give a recommendation one way or the other.
Twinkle works absolutely fine for me on Linux.
I'll assume that the Linux version of SJPhone wasn't suitable then?
X-Lite is great for a free solution. If you're willing to part with some cash for a license then eyeBeam is reasonably priced.