Regional Screenshot Capture: Freeze Screen Before Capture?
Don't know any applications that do what you're after but here's a couple of alternative options for you. Second two won't pause the video but will give you instant capture.


Pause the video then capture at will? Sorry if that sounds too obvious! Guess that you might be viewing live footage so that might not work...
Shift+command+4 will give you a regional marquee grab (takes grab on mouse-up) saved to the desktop (or add the control key to add to the paste buffer). 
Use Grab - its the OSX 10.6 native screen capture app under /applications/utilities. It has the option to grab the current window (or selected region or entire screen). 


Choose the option you want
Make the screen ready (ie video playing)
Choose it with a click
You may be able to set this up as a keyboard shortcut under /system preferences/keyboard/keyboard shortcuts - I've not tried.



I hope that helps!