The post on game theory reminds me to recommend the 12manage site for (mostly) authoritative explanations and discussions on a wide range of management topics, including strategic management – multiple languages too! Wikipedia does OK within its own constraints, but as I’ve blogged before, it’s pretty flakey on strategy. Jaap de Jong who edits 12manage does a great job managing and moderating the content and discussion to keep it on track, though it’s not possible to prevent entirely some off-target comments getting through.
http://www.knowledgebrief.com is another great place for information on business management concepts and theories. In a slightly different form, each concept is written with exactly the same sections as each other in a consise format making comparison very easy. Is also all written by doctoral researchers and experts in each individual area, so you can be assured of its quality. Designed to access the essential information quickly, it also has recommeded further reading in both print and websites for all the detailed information you could need.