Here is his page at the Concise Encyclopedia of Economics.
Alchian is not an economist that has influenced me directly, so I don't feel equipped to say much about him (there will be thorough commentary elsewhere, to be sure).
His promoters always seemed to think his ideas were more radical and a strike to mainstream price theory than they ever seemed to be to me, but perhaps exactly why they saw him that way will become more clear as others share their reflections.
His work is particularly focused on property rights, but he reminds me of someone like Samuelson because in addition to his big research agenda he has a lot of intriguing nuggets of ideas to dig into, like why discrimination would be worse in a heavily regulated industry or why firms ship their best quality products (assuming the product is the same but its quality heterogeneous) farther than their lower quality products.
UPDATE: Don Boudreaux - someone who knows Alchian well - celebrates something in him that I have also long celebrated in economics generally on the blog here. Don writes: "In short, each of these scholars [Alchian is one in a longer list] reasons and analyzes more as a great biologist reasons and analyzes — and not as a great engineer reasons and analyzes."
As I've often noted on here - the biology/economics analogy is dead on. And it stands to reason, of course. A human is just a smart ape after all and we ought to expect that good conduct of the study of human social relations bears a striking resemblance to biology. His "engineering" point is a little forced... biological engineering has given us the healthy and long lives that we enjoy in modern capitalism, after all. But it's in the right spirit in that good economics that considers policy bears little resemblance to the engineering of physical systems.