[atomack]

Maybe I'm a luddite but I used books.

Any book on common practice harmony will help with most pop/rock and classical up to 20th century. I used Harmony by Piston and happily recommend it. For some jazz theory The Jazz Piano Book by Levine was good for me.

[abruzzi]

Agreed. I loved Piston, with the caveat that his book is about the "Common Practice Period". It will teach you the standard approach to harmony used in the 18th and 19th century. But its a grounding that helps to make all the post 19th century developments in harmony make sense.