Never found anything valuable, though. People actually tend to overprice collectibles like cards, comics or coins. Rare books are a field requiring knowledge and truely rare books, which you don't see at garage sales. I can see old video games, but you still don't see MS in-box Dragon Warrior III games that often, either... although SNES games like Final Fantasy III are probably more common and sell in the $50-80 range as well. Battery life of only 10-15 years is always a concern with these games, too.
Older NES and SNES RPGs and others sell for a pretty penny. I've never seen Atari-system games go for much of anything.
DW and FF games for the NES can sell for upwards of $100 in boxed MS with functional battery, as can SOM and CT for the SNES. Moreover, old rarities such as Mrs. Pacman have always been desirable for no other reason than low production.
I have never followed Sega, but I'm sure their counterparts to DW/FF pull similar amounts.
I look forward to the day when we have SNES FFIII-type games in Flash/Java-format on the WWW.
If you ever see a Texas Instruments Speak and Spell that works pick it up! I found one for .25 at a garage sale and sold it for $80. They're desirable because of the sounds they make. i guess they use the electrical components for sound effects and stuff. Who knows?!