Pampers.com and Huggies both have reward programs for buying their products (I know, not free), but if you search for either in google, you can find free points to start your account even before the babies come.
The formula companies send 'checks' for their products, the dipaer co send coupons, gerber also send coupons.
Make sure they register at the baby stores. Target, Buy, Buy Baby and BRU all give 'goodie' bags when you register that have coupons and some samples in them (Targets was a really good goodie bag, followed by BBB, then BRU).
CONGRATS!