If you are already familiar with camtasia maybe remote accessing the linux desktop from a machine running camtasia would work best. Probably no current native linux apps are near it's equal. This is based on camtasia's reputation, not personal experience.
Two vnc based linux apps I have worked with are screenkast and pyvnc2swf.
Screenkast outputs it's own format (.isd?) making it unusable without the built in viewer and it doesn't support audio. This is based on the last time I tried it. Possibly the export to different format (flv, avi, etc) feature is working now.
Pyvnc2swf is simple to use and doesn't require installation. Supports audio. Outputs swf and has some editing tools. Conversion to other formats is possible but not exactly intuitive to say the least.
xvidcap. If you see a linux screencast on youtube etc there's a good chance it was done with this. Many features.
There are a few more. Google is your friend on that part.