Engineering 

Products and manufacturers Buyer's Guides, databooks, and catalogs are very helpful to engineers.

More and more of these catalogs are available online, making it easier to find parts and components, tools, equipment, and corresponding specifications.

Global Spec

• The SpecSearch system contains 24,000+ company product catalogs -- 2.3 million+ products and 177 million+ searchable product specs
• For other services they provide, read the "About" page

IC Master (free registration required)

• IC Master is a database of currently available integrated circuits. It includes more than 100,000 base part numbers, including 13,000 new device listings.
• From this site you can review the latest IC product information, and identify and locate manufacturers and distributors.

McMaster-Carr

McMaster-Carr is a large on-line catalog of products for industry. It contains over 410,000 items including:

• Hand tools
• Hardware
• Storage equipment
• Springs
• Protective clothing
• Drills and drill bits
• Pumps
• Piping and tubing
• Plumbing equipment
• Measuring, leveling, and inspecting equipment and much more

Sigma Aldrich

• Search for over 200,000 chemicals and laboratory research supplies
• Site allows you to search by product name, CAS number, molecular formula, or do substructure searchres
• A print copy of the catalog is located in Science Reference on C-Level, with call number TP202.A38

ThomasNet (free registration required)

• The online version of the Thomas Register lists ~160,000 manufacturers of industrial products and services
• The library also has a print edition, T12.T6, in the General reference collection on M-level