This portion of the USCIS web site provides links to PDF copies of the forms used by USCIS for citizenship and immigration.  The most frequently-requested USCIS forms at this site include:

Apply for Citizenship (Form N-400)
Employment Verification (Form I-9)
Apply for a Green Card (Form I-485)
Help My Relative Immigrate (Form I-130)
Affidavit of Support (Form I-864)
Renew or Replace My Green Card (Form I-90)
Remove Conditions on a Green Card through Marriage (Form I-751)
Apply for Employment Authorization (Form I-765)
Apply for a Travel Document (Form I-131)