More and more free multi-party video conferencing applications are readily available over the Internet and both Server-to-Client (S/C) or Peer-to-Peer (P2P) technologies are used. Investigating their mechanisms, analyzing their system performance, and measuring their quality are important objectives for researchers, developers and end users. In this paper, we take four representative video conferencing applications and reveal their characteristics and different aspects of Quality of Experience. Based on our observations and analysis, we recommend to incorporate the following aspects when designing video conferencing applications: 1) Traffic load control/balancing algorithms to better use the limited bandwidth resources and to have a stable conversation; 2) Use traffic shaping policy or adaptively re-encode streams in real time to limit the overall traffic. This work is, to our knowledge, the first measurement work to study and compare mechanisms and performance of existing free multi-party video conferencing systems.