The next frontier in Big Data Analytics beyond Hadoop is Real-time processing. The two open source leaders in this space are: S4 from Yahoo! (now in Apache Incubator) and Storm from Nathan Marz (of BackType and Twitter).
After some research on this topic, I found only a couple of good articles comparing the two, and a question on Quora. The links are given below.
S4 vs Storm
- Awesome blog post from Gianmarco de Francisci Morales: http://gdfm.me/2013/01/02/distributed-stream-processing-showdown-s4-vs-storm/
- Slideshow from Richard McCreadie of the University of Glasgow: http://demeter.inf.ed.ac.uk/cross/docs/s4vStorm.pdf
- Comprehensive answer by Divye Kapoor to a Quora question on this topic: http://www.quora.com/What-would-you-choose-between-Flume-Yahoo-S4-and-Backtype-Twitter-Storm-and-why
In addition to S4 and Storm both of which are open-source, there are other commercial products in this space such as InfoChimps Cloud::streams .