Everyone seems to have covered most things... I also like to have a strainer on hand. Very useful for rinsing substrate, not to mention catching all the crud and debris when I pour my old water down the bathtub drain.
And now for some other random items I use all the time:
-A pipette. Basically its like a mini turkey baster. Very handy for feeding live/frozen foods, not to mention taking water samples (I can NEVER get exactly 5 mL by dipping the test tube into the tank). I have gotten them from pet stores but technically they are for use in microscopy (for putting water onto a slide).
-A baby syringe. Very handy for measuring meds, ferts or water treatments when your tank is so small that you are measuring water additives in mL. You can get them at drug stores... personally I get mine from the vet as its what they send my dog's dewormer home in.
-A slurpee straw. You know the ones with a pseudo-spoon at one end? I love them for scooping out uneaten food without getting my hands wet.
-A plastic cup. Either disposable or a cheap plastic cup meant for patio use. I use it to keep my net, pipette, syringe and slurpee straw in :)