Homebrewtalk.com – Has great forums where you can find answers (although look for a number of people agreeing since there isn’t an editing process) to issues, great recipes, and how-to’s for building equipment.
How To Brew – John Palmer is very knowledgable about homebrewing and you’ll see his name pop up all over. This book is a great resource on how to brew as well as why those steps are necessary and what is happening (i.e. during the mash). The entire first edition of this book is available online.
Northfield, MN’s water profile – Less important for extract brewing than all-grain but because Northfield’s water is so hard, it’s important to take it into consideration. (Here is the most up-to-date water report despite being from 2012)
I use both Beersmith and John Palmer’s Excel file to create the water profile for brewing. The easy rule of thumb, would be to use 1/3 Northfield water, 2/3 RO water (available at Just Food Coop where you can bring a container to fill), and a pinch of sea salt. For more exact calculations, here are filled-in Excel files for three types of beers: An IPA, a Saison, and a Stout.