Handbooks are collections of data and procedures that help you find quick information from authoritative sources.
Handbooks can be print-based, or increasingly, electronic searchable databases. This enables you to search across many sources at once to locate a particular piece of information.
Knovel - a searchable collection of engineering texts and handbooks from a variety of publishers. the Knovel format allows searching by particular property, for example.
EngNetBase - A collection of resources from the publishers of the CRC Handbook of Chemistry and Physics. Over three thousand titles in all areas of science and engineering are searchable through this resource.
Other Important handbooks
Safari - Collection of Programming and Software Guides
ASM Materials Information Online -- Materials properties including alloy and phase diagram information
CES Edupack Materials Selector -- Materials properties information, with a graphical searching option
Wolfram Alpha has been touted as a one-stop clearinghouse for scientific information. It has a very sophisticated search engine that translates a question into a search for technical and/or mathematical data.
The one challenge with Wolfram Alpha is the lack of referencing to original sources of information. A typical handbook will provide references to primary literature, while Wolfram Alpha provides links to a 'class' of data sources, rather than an individual source. For most quick-lookups, this will be sufficient, but for detailed research, it is often important to know under what conditions an experiment took place, to make sure that the data you find is applicable to the project you are working on.