Internal Revenue Service
You can find federal income tax forms and answers to most of your questions pertaining to federal income taxes at this site. You can also visit this site if you want to check the status of your federal refund.
Board of Equalization
You can find California state sales and use tax forms at this page. You can also check sales tax rates for the different counties and municipalities, and file your sales tax returns online.
Franchise Tax Board
You can find California state income tax forms as well as answers to most of your California income tax question at this site. You can also check the status of your state income tax refund.
Social Security Administration
This site provides information pertaining to Social Security. Certain employment forms, such as W-2's are available here as well.