Kaggle They make data science a sport, enough said.
DataKind DataKind may not technically be a startup because it is a nonprofit, but they are doing cool stuff. They match nonprofit organizations with people that love to analyze data and create visualizations.
Cloudera They call themselves “The Platform for Big Data”. They are working hard to make hadoop easier to use.
Coursera Coursera is an education startup, but with 2 Computer Science Professors as founders, you can bet they are crunching a lot of data about how people learn.
BigML They are trying to make machine learning available to everyone. Machine Learning as a Service!