Fairly advanced site with explanation and online exercises.
English for Everyone
A series of worksheets to practice
Worksheet with explanation and exercises
Very comprehensive explanation of using articles.
Same site as above, focusing on count/non-count
Online quizzes, with an option to print in handout format. Nice supplement. Scroll down for exercises for articles.
Commnet count and non-count nouns
Great explanations, although a little advanced, with quizzes at the end
ESL Library count/non-count handout
Printable overview with exercises
English for Everyone count and non count noun worksheetsCouConCC
Worksheets for basic count and non-count identification as well as few/little andtoo/very