Ethnologue Report for Hausa: Information about where Hausa is spoken and the number of its speakers.
Hausa Language: Wikipedia article.
Hausa Page: Guide to books, courses, software, and audiovisual resources for studying the Hausa language, from a distributor in Melbourne (Australia).
Teach Yourself Hausa: Introduction to the Hausa language, including the basics of its phonetics and grammar, and some sample sentences illustrating everyday conversation in the language.
The Hausa Language - by Paul Newman: Write-up on a reference grammar of the Hausa language, by one of the top authorities in the field, and published by Yale University Press.
UCLA Hausa Page: UCLA resource guide to the Hausa language and the culture of Hause speakers. Includes an online Hausa grammar.