SPDY was originally developed by Google to improve the delivery of content in the web browser.
Google Chrome already supports it and it is used by several Google properties.
Mozilla Firefox 11 users would have to enable it manually for the time being:
Open about:config and set network.http.spdy.enabled to true
Mozilla spoke about their implementation of SPDY:
Google did a lot of work to launch this technology and to evolve it in the open, but it isn’t a Google only project any more. Since the implementations in Chrome and various Google web services were introduced we have seen either code or commitments regarding SPDY from many other products and groups including Amazon’s tablet, node.js, an Apache module, curl, nginx, and even a couple CDNs along with Mozilla. In my opinion, that kind of reaction is because engineers have looked at this and decided that it is solves several serious problems with HTTP’s connection handling and that this is a technology well positioned for us all to cooperate on. There is also discussion and preliminary movement in all the right standardization forums such as the W3C TAG and the IETF. Open standardization of the protocol is a key condition of Mozilla’s interest in it, but it is not a precondition to using it. Gathering operational experience instead of just engineering on whiteboards, is a valuable part of how the best protocols are made. The details of SPDY can be iterated based on that experience and the standardization process. The protocol is well suited to that evolution at this stage.