I've decided that egg cartons sort of suck for starting seeds. They are fine for germinating seeds-- but I found that I had to transplant my seedlings pretty much as soon as they sprouted. They're just not deep enough for seedlings to develop a proper root. Since I germinated most of my seed in baggies, the egg cartons were sort of a waste of time. Everything I started in egg cartons has been transplanted. Above, I have 6 inch high extra-large party cups with a drainage hole in the bottom. I've been using these for anything that develops a tap root... like Echinacea and Echinops. I can fit 18 of these in a standard size drainage tray (purchased at the garden store). The only drawback is that they tend to fall over when you move them.
For new starts, especially things that germinate easily or have tiny seeds that make baggy-starting inefficient, I've bought some commercial seed-starting trays. They're pretty cheap, and I've found a few places that stock they year-round (unlike the big-box garden stores which look at you like you're crazy when you ask for seed-starting trays in the middle of the summer).
I started these Yarrow seeds in plastic mushroom trays. The Yarrow "Cloth of Gold" seeds are ridiculously vigorous... they started in 24 hours, and I obviously sowed too densely. On the other hand, the Yarrow "Mia" seeds were a lot less vigorous.