SPDY is a protocol similar to HTTP with some optimizations to reduce latency and is the basis of HTTP/2.
The design of SPDY requires SSL, which is a barrier to entry, enforcing it as a secure protocol. It also has compression (gzip or DEFLATE) by default so that it is always fast. These two design principles are very opinionated, unlike HTTP which can (and often is) be used in inefficient and insecure ways.
Community members that have deployed SPDY support on their primary personal website.
SPDYCheck can check a website for support.
Impact on best practices
Once SPDY or HTTP/2 is in all modern browsers, best practices may no longer be accurate due to the way the protocol has built-in minification and concatenation. Until then, we should probably still be minifying and concatenating so that browsers that aren't supported will still be quick.