(Revisiting this topic with the release of the Kindle DX)

Having owned both generation Kindle readers and now getting a Kindle DX; I'm very excited for true PDF handling on an e-ink device!

An image of _Why's book on my Kindle (from my iPhone).

This gives me a device capable of storing hundreds of thousands of pages that are full text search capable in the form factor of a magazine.

What references (preferably PDF to preserve things such as code samples) would you recommend? Ultimately I would like reference material for every modern and applicable programming language (C, C++, Objective-C, Python, Ruby, Java, .NET (C#, Visual Basic, ASP.NET), Erlang, SQL references) as well as general programming texts and frameworks (algorithms, design patterns, theory, Rails, Django, Cocoa, ORMs, etc) and anything else that could be thought of.

With so many developers here using such a wide array of languages, as a professional in your particular field, what books or references would you recommend to me for my Kindle? Creative Commons material a plus (translate that to free) as well as the material being in the PDF file format. File size is not an issue.

If this turns out to be a success, I will update with a follow-up with a compiled list generated from all of the answers.

Thanks for the assistance and contributing!

UPDATE

I have been using the Kindle DX a lot now for technical books. Check out this blog post I did for high resolution photos of different material:

http://www.matthewdavidwilliams.com/2009/06/12/technical-document-pdfs-on-the-kindle-dx/