I personally love the little 3-oz Dixie cups for collecting seeds. I buy them anyway for taking medicine (things I have to take with milk) because I get tired of washing glasses.
I rinse them out after use and let them dry, then use them for seed collecting (and occasionally seed starting!) They're nice because you can write on them, and if the seeds are small (like petunias) you can stack them to save space. I store my small seeds in glassine envelopes tucked inside larger paper envelopes that I make from small mailing envelopes. They fit nicely in plastic shoeboxes.