Here is a list of tutorials for learning how to write MapReduce programs on Hadoop, the opensource MapReduce implementation with HDFS.
The official tutorial on Hadoop MapReduce framework: http://hadoop.apache.org/docs/r1.0.4/mapred_tutorial.html.
Yahoo! Hadoop Tutorial
More about MapReduce
To better understand the design behind MapReduce, it is always good to read Jeff Dean and Sanjay Ghemawat’s MapReduce paper: http://research.google.com/archive/mapreduce.html.