Facebook and twitter are the obvious big boys. Google plus is definitely worth having especially if you rely heavily on your site ranking high in google which most people do. My guess would be Digg is next in line, but that's just a personal preference.
I'm afraid I disagree, the coding community do use Facebook (who else wrote all those apps?)
I'd say that the networks to have Facebook, Twitter, Linkedin and Google+
I personally don't like Facebook, or Twitter, but wouldn't exclude them from 'share' buttons.
The most relevant for you I guess are Twitter, LinkedIn and social bookmarking sites. I personally use Twitter, FB and Google+. All in all, the social media buttons you display should match the sites you are most active on.