Programming Texts and Tutorials is an excellent resource about anything you ever wanted to know on the subject of UNIX programming in general, and Linux in particular. It covers a broad range of languages, protocols and software. Everything from C and Perl to Gimp and Apache. Worthy bookmark indeed.
Update (July 17th, 2016): Since the origin website to which I linked went down, here is an alternative collection of wisdom for all sorts of programming languages and technologies. Enjoy!