Every photographer, from the family record keeper, to bloggers like me shooting an actor on a Red Carpet, or a bird watcher photographing that rare species they finally found in the wild, or a freelance or staff photographer, can improve, no matter what level they are working at now. One terrific resource I came across recently is the gifted and innovative Joey Terrill, who along with an impressive career as an advertising and editorial photographer, also generously shares his time with many others seeking to improve.
Mr. Terrill has two excellent websites, the first, his professional photographer's website, joeyterrill.com (which is a model of how a pro should represent themselves online). Then he has a second website dedicated to illuminating his vast collection of tips, techniques, and advice for the serious amateur or professional: penumbraproject.com. I would recommend both to anyone interested in lifting their photography to new heights.
Joey also recently mentioned to me two other very helpful resources: strobist.com and daveblackphotography.com -- two excellent websites also dedicated to helping others thrive in their photographic endeavors.
We live in an increasingly fast-paced, visual world; and those who can produce compelling, high-quality images (or hire someone to do so for them) will always do better than those who cannot.