Which libraries do you use for machine learning for C/C++, Python, Java?
Weka (in Java) is definitely one of the most popular, and has been exposed to most other (including Python).
For C++, I recommend looking at Shark, but Shogun is also quite good. (see, also, this related SO question).
Weka is good. Rapid Miner is more comprehensive and more complicated.
Python related:
Orange: data mining framework
NLTK: natural language processing and text analytics
Gensim: Vector Space Modelling
Machine Learning: An Algorithmic Perspective: Although it's a book, we can touch the source code directly.
NumPy/SciPy related
Python related Machine Learning Library
If you really need a tons of Machine Learning packages for review. MayBe http://mloss.org/software/ is the place.
For Python:
MDP is a signal processing library, and allows you to easily build pipelines of algorithms
scikits.learn is a rapidly expanding collection of machine learning algorithm