Karadzic's arrest happened with a tip from an unknown foreign intelligence source to Serbian authorities, who then began to monitor and eventually arrest Karadzic.
There are others who, while not directly responsible for his arrest, indirectly contributed so much to his capture. I want to mention three, in no particular order:
1) Carla del Ponte. The Swiss former chief prosecutor of the ICTY was almost solely responsible for the world's attention on Karadzic during the last few years, when he might have otherwise been forgotten. Karadzic was relevant to Serbia, the UN, and the West solely because of her.
2) Nick Hawton. This intelligent, savvy British journalist lead the Balkan beat for the BBC for several years. The world's lifeline as to what was going on with Karadzic is due largely to Nick.
3) Natasha Kandic. This Serbian human rights activist is largely responsible for changed attitudes in Serbia about Karadzic. In 2005, at great personal risk, she released a 1995 video from Srebrenica that corroborated allegations of genocide. Had the Serbian people not been so persuaded, yesterday's arrest may have never garnered the political will to succeed.
Thanks also to the brave men and women who put their lives on the line to make yesterday's arrest.
So many other people who were involved for years with Karadzic and deserve recognition: SM, TF, AL, RG, KJ, YG, RS, MT, JS, JS, FH, LE, Jinx, Shaina and many more who I hope to credit soon.