Despite my initial skepticism...
These things rock!
They come in a set of 6 different sizes and work just like... well... just like a shower cap.
I literally use them at least once a day and it has been great not to fall back to saran wrap every time I turn around. The picture shows my veggies and broth which I prepped early in the day for chicken pot pie covered in their cute "caps" and ready to go.
For other thrifty green tips check out the Green Baby Guide's Thrifty Green Thursday.