I work in Video Conferencing and we use Polycom RealPresence or Mirial Clearsea for professional connections such as H.323 or SIP. RealPresence is free and can work as a standalone client or can be linked to a CMA server - it's pretty good and free so thats what I would recommend but, it is missing a few things like FECC and AES support currently. ClearSea supports more features and is a free app but to call H.323 or SIP sites (that aren't other Mirial clients themselves) it costs £20 or so per month. Not a bad option - especially if you require encryption or anything - but, it obviously costs more compared to RealPresence.
For personal use or just video chatting with family/friends I'd recommend Skype. Its solid on video and audio and works on almost any device or computer out there.