The term alpha geek, has got most of its currency through a talk and online essay by Tim O'Reilly. In this talk he says that by watching what alpha geeks are doing you get a sense of what the future is like for the technology industry as a whole.
Alpha geeks are typically experimenting with technology. In software development they are the people who first try new languages and frameworks. They are also inclined to build things to solve their problems if there isn't a viable alternative out there. Although they can be wedded to a particular platform, most of them will use what makes them most effective, hopping between platforms. They usually are very passionate about software development and spend more time, often spare time, reading books, going to conference, and tinkering.
Tim's view was that alpha geeks, as a whole, are leading indicators. This isn't true of an individual, who may try all sorts of things and drop if they don't end up being useful. But the aggregate result is important. If many alpha geeks are liking the same thing, then there's a good chance that this is something that's important. It may be several years before it hits the mainstream, but when it does it will matter.
I tend to agree with this view. Although most of my work is playing around with the recent past, trying to capture things we know in a comprehensible way, I do keep an eye on what alpha geeks are looking at and liking. So far its tended to lead to me things that are usually interesting, and more often than not to things that make an important difference to our profession as a whole.
In many ways the name "alpha geek" is unfortunate. In this context, at least, it doesn't necessarily indicate that the alpha geeks are the best technologists - someone who is very expert in a particular technology but doesn't look for new stuff wouldn't fit. "Alpha" also has connotations of superiority and dominance that also don't necessarily fit. The point in this context is that alpha geeks are the people who like to explore and try new things.