There are many ways, from the brute force anolog.
Downsample your Montior output to Composite or S-Video and Capture it on another Computer with a frame grabber.
Or Output your Monitor to HDMI (or convert your DVI -> HDMI) and re-capture that on a HD capture card on another computer.

But that's a mess, uses 2 computers etc...

Or if your using Jitter there is a option to capture a portion of your desktop as a Matrix source. That's a perfect solution, but only for Max/Jitter.

I used to use Camtasia which included a live output driver. I've been using a venerable old V3 version for years. But I recently had to purchase V6 to edit some files for a client.
And I was disappointed to find that they had abandoned the live feature in V6 (v5 still has it)

I've never used the other features of camtasia much and it's seemed like an expensive solution, but I could never find another software option like it.
However this go round I did find a source for what looks like a better and free alternative!

http://www.splitmedialabs.com/vh-video-sdk/vh-screen-capture

Actually the features and performance of this tool are awesome, way better, though just as confusing as Camtasia ever was.
The multi-mix option is a whole new feature set to explore. I've needed those exact features in the past. (say to get multiple camera inputs into a VJ app.) Isadora my favorite application supports multiple inputs, and native capture drivers so I don't need this feature as badly now but it's still nice to have in a bag of tricks.

For instance you can change the video settings, and even some effects on the video input source in real time.
Scale and mix multiple cameras. (need a picture in picture this will do it with more options than you could hope for.)
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