Machine Learning Startup Ideas
First of all, I would get acquainted with some core concepts of Machine Learning before building websites:


Common algorithms, differences between them (Naive Bayes, KMeans, Regression, Support Vector Machines, Decision Trees, you name it...)
Supervised vs. unsupervised
Features and feature selection 
Classification evaluation (precision, recall, accuracy)


Machine learning is a huge field which is applied in many different areas. I don't know how proficient you are at this stage, but I would certainly play around with these things first in order to get a solid understanding of what you're dealing with. After that, the options are virtually limitless in a world of Big Data (forgive me for using the annoying buzz-word). What is truly valuable is someone knows how to do machine-learning rather than what (because there's enough to go about).