Here’s a list of engineering resources around the net I’ve found useful. Updated frequently. Feel free to suggest any additions in the comments.
Archive.org Public Safety Codes: Collection of various building codes available for download. Particularly useful is every version of the IBC – 2000, 2003, 2006, 2009 and 2012.
Public.Resource.org Building Codes: Another online code collection, largely overlaps with archive.org.
PublicCodes.Cyberregs.com: Online code repository maintained by the ICC. Doesn’t allow downloading, and navigating is an extreme pain.
UglyBridges.com: User-friendly interface for National Bridge Inventory data.
Nationalbridges.com: Another online parsing of NBI data.
Bridgehunter.com: Database of notable bridges, past and present.
Structure Magazine: Articles on various structural engineering topics, available free (including all back issues). I particularly like their Antiquated Structural Systems series.
Structural Engineer: Another structural engineering magazine.
Modern Steel Construction: Articles focusing on steel construction. Current and back issues all freely available.
Whole Building Design Guide: If you build something on a military base, these are the codes that get used.
Army Corps of Engineers Manuals: Design manuals on a variety of engineering topics.
The Earthquake Engineering Online Archive (NISEE): Many publications about earthquake engineering. Membership costs a one-time fee of $25.
S.K. Ghosh: Articles on structural engineering and building codes, written by one of the premier engineers of our time.
Reed Construction Data: Economic and cost data for the construction industry, as well as a breakdown of the building code adoption by state.
ASCE Library: If you feel like paying an arm and a leg, there’s a large library of structural engineering articles available.
PCI Technical Resources: Back issues of the PCI Journal, load tables from every edition of the PCI handbook, and a variety of other precast design resources, all freely available.
ACI Free Downloads: Freely available publications of some of the more minor ACI Committees. There’s also a large library of articles from the various ACI publications, many available for free to ACI members.
AISC ePubs and freePubs: Technical publications from AISC, including journal articles and historical standards. Some require AISC to access.
NFPA Research: Fire statistics and reports from the NFPA.
Google Books: If it’s a hundred years old or so, it’s probably available for free on Google books. Lots of great engineering publications here.
Historic American Engineering Record: Prints and photographs documenting achievements in architecture, engineering, and landscape design from the Library of Congress.
FEMA Library: Library of freely available FEMA publications, including NEHRP publications.
Forums and Mailing Lists
ICC Code Forum: Discussion of the various I-Codes
Eng-Tips: Forums for various engineering issues.
SEAINT Mail Archive: Structural Engineer’s Association mailing list archive, going back to 1995.
DesignToEurocodes.com: Forum for Eurocode design.