A fantastic collection of Linux tutorials and howtos. Mostly focused towards setup, administration covering a wide range of linux distros. For someone experimenting with distros, this is a treasure trove
unix-tutorials.com
HowtoForge offers a comprehensive collection of quick howtos about almost every topic on Linux. Besides setup and administration , covers a few good topics on programming
Open-of-course has a introductory tutorial on Unix and Linux concepts. A good starting point if are just about getting familiar with Linux. Also a good Linux system administration guide available here