Sanskrit Self Study An introduction to Sanskrit Language in 64 self study lessons by Chitrapur Math.
Discover Sanskrit A concise study of the Sanskrit language.
A Practical Sanskrit Introductory by Charles Wikner. This concise introductory contains exercises for correct Manner of articulation.
Harivenu Dâsa: An Introductory Course based on S'rîla Jîva Gosvâmî's Grammar, a vaishnava version of Pânini's grammar (pdf-file).
Ancient Sanskrit Online from the University of Texas at Austin.