Lets see:
I always have an appropriate music playlist in the background.
We've used a dinosaur plush toy as an "apocalyptic hydra".
Last time my friend provided background music so that my bard NPC could actually sing what he had to say to the characters (amazingly enough, it was not that hard to improvise while staying on beat)
Then we used a small parrot as a Roc in the battleship combat, in which conveniently I also had ship props.
An player just emailed me a letter she wrote to an important NPC, so I put it in Word with some hand-script font and printed it so I can use it next game session.
Another player brings a plastic warhammer to the sessions. It's awesome.
I bought a $1 wooden puzzle cube at the dollar store, gave it to the wizard player, and told him I would tell him what he found next session if he had solved the puzzle
I used a scarf once as a shawl.
I think that's all.
In fact I am putting a scarf in my DM bag so that I can use it next week. Check out this video of Robin Williams putting it to good use to impersonate different characters (Starts at 3:15):
[ame="http://www.youtube.com/watch?v=IL2Iv-kbc68"]Robin Williams on Inside the Actors Studio - YouTube[/ame]