as i've found myself doing more editing these days for podcasts, screencasts, whatever, i've been using a mix of tools. i've been really liking camtasia lately for most as it has provided me with what i've needed...after all i'm no professional.
one thing that it wasn't doing for me was allowing me to put overlays on top of video frames. actually it does, but i was just an idiot looking for it in the wrong place. camtasia calls these "custom callouts" and are hidden underneath the add callout features. i wouldn't have thought to look there as all the callouts i've seen basically are used for arrows, balloon boxes, etc. but nonetheless i found them and it does exactly what i wanted for my scenario. perhaps it will add value for yours as well so i recorded a rough video (no editing, sorry for the audio and super loud keyboard clicks, i lost my headset, new one en route) demonstrating where this feature is. it's useful for me, so i thought i'd share. of course other video editing tools do this for you as well, but if you aren't a pro (or like me want the one-click, produce to many file formats) and have camtasia, this may be handy.
hope it helps.