You can find below a list of resources which i have found very useful to get familiar with the special and general theories of relativity.
Special Relativity

Web
 http://www.einsteinonline.info/ Max Planck Institute's web portal which provides information about Albert Einstein's theories of relativity and their coolest applications, from the smallest particles to black holes and cosmology.
 http://www.phys.vt.edu/~takeuchi/relativity/ Special Relativity Lecture Notes of Department of Physics, College of Science, Virginia Tech.
 Einstein Light: A brief illumination of relativity. Multimedia presentation of the basics of special relativity  includes many helpful animations.
Books
If you are interested in the subject of space and time, and in special relativity, then this is a must read.
Brian Cox and Jeff Forshaw do a superb job of explaining Einstein's famous formula and theories in a way that is as easy and enjoyable to read as possible with very low content in mathematics.It is mostly focused on special relativity, with the last chapter introducing the general relativity.
The book to read once you are familiar with special theory of relativity. Here we go through the mathematics (to paraphrase Euclid, there is no royal road to relativity  you have to do the mathematics). But we are taken gently by the hand, with even a crash course in foundation mathematics for those with minimal mathematical background. Chapter 1 to 3 are dedicated to special relativity, the rest of the book exploring general relativity in deep details.
Although unanimously criticized for its bad translation, this book allows one to follow Einstein's actual thought process in arriving at these theories.
Reading the words of the master is an experience that should not be missed, even if sometimes some calculation details are left out during the demonstration.
Hopefully the reader will find some help in our articles, as they are supposed to go step by step.
You can find the book in html and pdf format at this location
General relativity

Web
 Reflections on Relativity by Kevin Brown Very complete presentation and stimulating thoughts on Relativity
 John Baez' general relativity tutorial : Easygoing introduction to general relativity  including formulae and mathematical concepts
 Introduction to General Relativity by Gerard 't Hooft
Books
A General Relativity Workbook is a textbook intended to support a onesemester undergraduate course on general relativity. It's very well written and Moore has a gift for language that few other scientists have.Through its workbookbased design, it enables us to develop a mastery of both the physics and the supporting tensor calculus.
The mathematics is introduced gradually and in a completely physical context.
A must have!
One small regret however: the unavailability of solutions to the Exercises and/or Problems
Einstein

Web
The collected papers of Albert Einstein: Princeton University Press presents The Digital Einstein Papers, an openaccess site for The Collected Papers of Albert Einstein, the ongoing publication of Einstein's massive written legacy comprising more than 30,000 unique documents.