I use the clear plastic tubs you get with almost every bakery purchase at the grocery store. I had (I have a ton that I keep reusing) people at work save them for me over the winter and then after a wash I fill them with seed starter and my seeds. The locking lids give them a nice greenhouse and when they get bigger I just pop off the lid until it's time to move them outside.
This year I also tried one of the clear plastic egg cartons. I'm not as keen on that one. The seedlings get too big for it too fast, and I had to move a bunch already. They're in water bottle greenhouses.
Thanks to a steam bath from an exploded hot water pipe my seedlings are huge already (I would not recommend this!
). I've started several tomato varieties, cucumbers, jalapenos, three types of lettuce, broccoli, and peppers this way. My wax peppers are the only ones that have not started yet.