Once you've exhausted all the usual suspects (MLA Bibliography, JSTOR, and of course the Rush Rhees and Robbins Library stacks), you can turn your attentions to more specific databases. These resources will help you find overview articles, quick facts, and definitions, as well as scholarly articles.
International Medieval Bibliography (UR online)
Oxford Bibliographies Online (UR online)
Oxford Dictionary of National Biography (UR online)
New Catholic Encyclopedia (UR online)
Encyclopedia of the Middle Ages (UR online)
Mirabile: Digital Archive for Medieval Latin Culture (in Italian, but navigable)
These resources feature collections of primary sources online.
Monumenta Germaniae Historica (in German)
Patrologia Latina (UR online)
Dictionary of Old English Web Corpus (UR online)
Early English Books Online (EEBO) (UR online)
A completely non-comprehensive list of blogs that I have read and enjoyed. Join the conversation on medieval topics!