DynamicPageList is the single most useful extension that I use. It creates mainly list of pages satisfying various criteria (e.g. category, notcategory, linksfrom, linksto, title, uses, usedby, namespace). For more information you can read its extensive manual.
Labeled Section Transclusion allows selective translusion of marked-off sections of text. When it is used in combination with DynamicPageList you can synthesize the existent wiki content and create new dynamic pages according to specific criteria.
MathJax provides an easily installed way for typesetting LaTeX formulae in MediaWiki pages.