Using dimensional lumber (2x4 and the like) from the home center or lumber yard is a great way to save money on a project. Provided, of course, that you get the right stuff. Anyone who has walked down the aisles of the home improvement store and pick through the piles can attest: some of this stuff is downright funky. And we often pick the best of the bunch, only to find it twisted, warped, or curved once we get the wood home and it's had time to acclimate to our workspaces.
This guide from Pat Doherty uses real photos from the lumber yard to help you see the differences between flat sawn, quarter sawn, and rift sawn lumber. He helps you identify pith (bad), heart wood vs. sap wood, and the like. The knowledge comes from the 500-ish page Wood Handbook: Wood as an Engineering Material published by the USDA, but distilled into simple photos that help you identify grain patterns.
It's a worthwhile look. Just be sure to click on the actual photos to see the comments and notations.
A Photographic Guide to Selecting Lumber [Instructables.com]