Assorted tools, the best being a 1/2" drive torque wrench, lots of pliers, vise-grips, occassional wrench.
Unopened beer or liquor, often along rural roads where it was tossed out the window at night by an underage person when they noticed they were being followed by the cops.
Assorted clothing, found a Subway restaurant employees cap yesterday. The only other item of clothing I ever bothered to pick up was a nice windbreaker that was lying along the highway clean and dry.
Perhaps the most interesting was a youth sized Diamondback MTB in excellent condition. It was obviously dumped/abandoned so I contacted the local PD and took it in. It wasn't claimed within 60 days and was donated to a local charity.