There are plenty of SIP softphones on the market: some commercial other free. Although free the majority of free softphones are locked in a particular SIP network forcing you to download and install a different SIP client for each network that you are using.
Fortunately 2 nice free and unlocked SIP softphones* are available on the market. Both can be registered with more than 1 proxy making them very useful in some situations.
X-Lite ( http://www.counterpath.com/ )
This software is available from CounterPath (former Xten) as a free download. It is the smaller variant of the Pro version. The latest product in softphones family produced by CounterPath is called “eyeBeam”. The features available in “eyeBeam”, makes the upgrade from X-Lite to the commercial version worth every penny.
There are many versions of X-Lite and eyeBeam available on Internet on different sites. Some of them are customized for certain companies and they cannot be used outside their networks.
Express Talk (http://nch.com.au/talk/index.html )
This is a less known SIP client but with some nice features which may convince you to try it. The interface is windows like but very simple to use.
It has nice features such as recording a call, multiple lines and good sound quality.
Another interesting product it appears to be Cubix from https://www.virbiage.com/cubix.php. Besides SIP, Cubix is able to talk IAX as well. I didn’t do enough testing of this product so I cannot make any recommendation. Feel free to test it and communicate back your results.
* There may be others free and unlocked SIP clients available and not considered in this post. The reader who knows any other SIP client that meets these requirements is encouraged to leave a comment.