I've started to use the plastic trays that mushrooms come in for seedlings. They are brilliant because the plastic is quite thick - actually better than a lot of cheap seed trays you buy! The big ones that hold nearly a kilo of mushrooms are great for stacking in loads of loo rolls or peat pots, and the small ones are just the right size for a large batch of seeds. I just poke drainage holes in the bottom with an awl.
This got me wondering how many other things I currently throw away that could be reused in the garden...