Here are the areas I've been researching, some things I've read and some open source packages...
Nearly all text processing starts by transforming text into vectors: http://en.wikipedia.org/wiki/Vector_space_model
Often it uses transforms such as TFIDF to normalise the data and control for outliers (words that are too frequent or too rare confuse the algorithms): http://en.wikipedia.org/wiki/Tf%E2%80%93idf
Collocations is a technique to detect when two or more words occur more commonly together than separately (e.g. "wishy-washy" in English) - I use this to group words into n-gram tokens because many NLP techniques consider each word as if it's independent of all the others in a document, ignoring order: http://matpalm.com/blog/2011/10/22/collocations_1/ http://matpalm.com/blog/2011/11/05/collocations_2/
When you've got a lot of text and you don't know what the patterns in it are, you can run an "unsupervised" clustering using Latent Dirichlet allocation: http://www.cs.princeton.edu/~blei/papers/Blei2012.pdf http://www.youtube.com/watch?v=5mkJcxTK1sQ
Or if you know how your data is divided into topics, otherwise known as "labeled data", then you can run "supervised" techniques such as training a classifier to predict the labels of new similar data. I can't find a really good page on this - I picked up a lot in IM with my friend Ben who is writing a book coming out next year: http://blog.someben.com/2012/07/sequential-learning-book/
Here are the tools I've mostly been using:
Vowpal Wabbit (classification and LDA, poor documentation, C++ high performance): https://github.com/JohnLangford/vowpal_wabbit/wiki
Gensim (LDA, vector similarity, text processing, python): http://radimrehurek.com/gensim/index.html
Mallet (classification and LDA, java): http://mallet.cs.umass.edu/
Lingpipe (text analysis, clustering, classification, linguistics, java, commercial open-source): http://alias-i.com/lingpipe/demos/tutorial/read-me.html
Mahout (Hadoop, classification, clustering, LDA, collaborative filtering, java): http://mahout.apache.org/
Langdetect (language detection, java): http://code.google.com/p/language-detection/
Some blogs I like:
http://thedatachef.blogspot.co.uk/
http://www.machinedlearnings.com
MetaOptimize Q+A is the Stack Overflow of ML: http://metaoptimize.com/qa
The Mahout In Action book is quite good and practical: http://manning.com/owen/