There are a TON of great resources out there for frugal Mommas! Here are just a few of my favorites:
BabyCheapSkate is a great website for hot deals on baby stuff, it's updated regularly and also has things like diaper reviews, a diaper cost chart, formula buying resources, cloth diaper info, a great go-to place for deals!
We couldn't have managed without the Baby Bargains book, it's like Consumer Reports but just for baby stuff, and super practical. They give a great run-down of what to and what not-to buy, as well as reviews that include info on safety, reliability, and usability, I used this as a template for registering for my baby shower and recommend it to every new parent!
For general couponing, I use afullcup.com, they have boards for deals at Target, Old Navy, and more, for those of us that don't have time to sort through all the coupons and find the best way to combine them for shopping, this site does it all for you!
Believe it or not, CelebrityBabyBlog is one of my faves as well, they review products and offer the occasional coupon code here as well, granted, most of the items on here are a bit high end.
I'm sure there's a ton more out there, feel free to post your faves in the comments, after a while I'll compile the list of recommendations!