This is the one I use and practice...
http://justinguitar.com/en/TE-001-FingerGym.php**** EDIT ****
3Aug12:
Well, crap! I just visited justinguitar.com to review and move on to the next stretch in the exercise and found that what I posted above was incorrect. That is, while Justin's lesson TE-001 is titled "Finger Gym" that's not the one I practice to increase my stretch (and dexterity and strength).
The correct lesson number is TE-007 and is found here;
http://justinguitar.com/en/TE-007-FingerStretch.php Amazingly, he calls it, "Finger Stretch Exercise". :D The other posted above looks worthwhile too and I'll get to it someday...
Sorry 'bout that!
"It takes a lot of devotion and work, or maybe I should say play, because if you love it, that's what it amounts to. I haven't found any shortcuts, and I've been looking for a long time."
-- Chet Atkins