OpenGL has gone through several verions over the years but it seems that many of the tutorials you'll find on the internet are dated and mostly cover the fixed function pipeline. I wanted to find more information about programable shaders (glsl), framebuffers, vertex buffers, and many of the other newer features found in later versions of OpenGL. So I've compiled a list of good tutorials and sites I've found that cover these topics.
Start here first. These tutorials are well written and clearly explain some of the details of OpenGL that seemed mystic to me before. I actually understand the difference between a framebuffer and renderbuffer now!
Another great site that covers a wide variety of topics including lighting, shadows, and parallax/normal maps.
These tutorials include the full source code which is very useful for tinkering.
Anton Gerdelan has several tutorials on his website as well as a book. He has a good overview of ray picking and cube maps.
This site is full of mind blowing fullscreen glsl effects. Everything runs in the browser and the source code live reloads. It even lets you tweak anyone's code and save your own changes.
OpenGL 4 Reference Pages
When you've exhausted the above sites this is the official reference for OpenGL. It has complete descriptions on all of the gl functions. There are probably many others but these gems have helped me immensely. Let me know in the comments if there are other useful sites I missed.
API Docs for multiple versions
This is a great site to get information on different versions of gl* function calls.