eCFR: Daily updated current version of the CFR, technically unofficial.

CFR from GovInfo (1996-present): Online version of the official print verstion of the CFR. Browse by year & title, search using the govinfo advanced search tab (select "Code of Federal Regulations" in the "Refine by Collection" dropdown menu), & update using the List of Sections Affected (LSA) volumes.

FederalRegister.gov (1994-present): Official website of the Federal Register. Read the current issue, search by term or citation, which you may filter by document type, or browse by term or agency in the Federal Register Index.

Federal Register from GovInfo (1936-present): Online version of the official print verstion of the Federal Register. Browse by year, month, & issue, search using the govinfo advanced search tab (select "Federal Register" in the "Refine by Collection" dropdown menu), & browse all of the Federal Register Index volumes by year.

Federal Register from the Library of Congress (1936-1993): Digitized version (PDF) of the print version of the Federal Register, volumes 1 to 58, browseable by volume.

CFR from the LII: Current version of the CFR, browseable by title, part, & section.

CFR from Justia: Current version of the CFR, browseable by title, part, & section.

Federal Register from Justia (2005-present): Browse by agency or date, search full-text by agency, document type, & date.

CFR from HeinOnline: Search or browse by year, title, index, or cite [WCL proxy link]. Update using the List of Sections Affected (LSA) volumes [WCL proxy link].

Federal Register from HeinOnline (1936-present): Browse or search from volume 1 to the present, updated daily [WCL proxy link].