APA (American Psychological Association)

Electronic Reference Formats (APA)
Using APA Format (Purdue)
Noodlebib for MLA and APA and Chicago/Turabian (UT Austin users)

Chicago Manual of Style/Turabian

Chicago Style Citation Quick Guide (Chicago Manual of Style Online)
Chicago Style: Works Cited or References (University of Wisconsin)
Turabian (Bridgewater State College)
Noodlebib for MLA and APA and Chicago/Turabian (UT Austin users)

MLA (Modern Language Association)

MLA Frequently Asked Questions (Modern Language Association)
Using Modern Language Association Format (Purdue University)
Beyond MLA: Documenting Electronic Sources on the Internet
Noodlebib for MLA and APA and Chicago/Turabian (UT Austin users)

Other Electronic Style Formats

More Style Manuals (University of Texas Libraries)
List of style manuals by subject
Citing Electronic Sources (Library of Congress)
Citing films, legal documents, maps, photographs, recorded sound, etc.
Citing Online Sources (The Linguist List)
Endnote
Software to organize references and create bibliographies