SPDY, the alternative to HTTP proposed by Google is slowly gaining strength. Granted, it's got a long way to go, but Twitter has started adopting it and using it by default, with browsers that support it.
SPDY, as the name implies, was devised to speed up internet connections.
Google is working on several fronts to speed things up, from its own apps to Google Chrome to redesigning HTTP with better optimizations in mind.
SPDY isn't really a new protocol. Rather, its creators tweaked HTTP in areas where it could be improved, particularly adapting it to the realities of today, where bandwidth is no longer much of a problem, and network is much more likely to get bogged down by things like latency and the sheer number of packets trying to get through.
The improved protocol has been around for a few years now, but trying to change something as fundamental as HTTP isn't easy and it's not going to happen overnight.
Google debuted SPDY in its own products, Gmail, Google Search and so on and built support into Google Chrome. Slowly, others started to adopt it, but Twitter is one of the largest sites, that doesn't belong to Google, to use SPDY.
Obviously, you'll only get SPDY connections to Twitter in Google Chrome. It also works in Firefox 11 and above, provided you enable the feature first. Go to about:config in Firefox 11 or 12, find "network.http.spdy.enabled" and set it to true.
Amazon is another big player to use SPDY, albeit in a much more controlled environment, it's enabled in the Silk browser, powering the Kindle Fire, for connections to the Amazon cloud, which pre-processes web pages requested by the browser.
There is work on standardizing SPDY, but web standards take years to be ratified and then a few more to become widely used. In the meantime, if you'd like to enable SPDY for your server, there's an Apache module which can help you do that.