Zoiper 2.0 Free (previously known as Idefisk) is a telephony application that provides basic and advanced VoIP functionalities at very low cost.
Unlike more popular VoIP apps, Zoiper Free supports supports more advanced telephony standards such as T.38 Fax over IP and Asterisk and SIP-capable IP-based communication systems and infrastructures. Zoiper definitely looks slick and offers things such as a contacts book, call history, videoconferencing, call forwarding and the ability to easily manage multiple accounts.
Zoiper integrates into Outlook and provides support for high quality voice codecs such as GSM, Speex, ILBC or 711th/u. The developers claim that this provides one of the highest quality audio you'll find on a VoIP app and prevents the annoying echo that affects many of them. The problems start when voice quality or call quality declines - its hard to tell whether this is a result of Zoiper or the VoIP client you're using it with.
Overall, Zoiper is an interesting product and could help improve your call quality although it's a bot of a hassle using it when Skype provides more or less the same quality voice quality and user friendliness.