There are several companies that offer GED study guides, both general and for the specific parts of the GED test. I have grouped the guides by publisher:
McGraw-Hill/Contemporary
McGraw-Hill's GED: The Most Complete and Reliable Study Program for the GED Tests by Patricia Mulcrone 
McGraw-Hill's Top 50 Math Skills For GED Success by Robert Mitchell and Dolores Emery
McGraw-Hill's GED Mathematics: The Most Comprehensive and Reliable Study Program for the GED Math Test by Jerry Howett
McGraw-Hill's GED Science Workbook by Robert Mitchell
McGraw-Hill's GED Social Studies Workbook by Kenneth Tamarkin and Jeri Bayer
McGraw-Hill's GED Language Arts, Writing by Ellen Frechette and Tim Collins
McGraw-Hill's GED Language Arts, Reading by John Reier
Contemporary's Complete GED: Comprehensive Study Program for the High School Equivalency Examination by Linda W. Nelson, Kathleen D. Mikkin, Suzanne E. Rausch, Janice S. Phillips, and Patricia Mulcrone
GED Language Arts Writing by Ellen Carley Frechette
Contemporary's GED Mathematics: Preparation for the High School Equivalency Examination by Jerry Howett
Calculator Power for the GED with Casio fx-260 Package
Top 50 Math Skills for GED Success - Student Text with CD-ROM by Robert Mitchell and Dolores Emery
Top 50 Reading Skills for GED Success - Student Text with CD-ROM by Judith Gallagher
Top 50 Writing Skills for GED Success - Student Text with CD-ROM by Tim Collins
Top 50 Social Studies Skills for GED Success - Student Text with CD-ROM by Robert Mitchell and Dolores Emery
Top 50 Science Skills for GED Success - Student Text with CD-ROM by Robert Mitchell
Steck-Vaughn Company (Houghton Mifflin Harcourt)
Pre-GED Complete Preparation by Steck-Vaughn Company, Mott S. Nolley
Pre-GED Mathematics by Steck-Vaughn Company
Complete GED Preparation by Steck-Vaughn Company, Ellen Northcutt
GED: Mathematics by Steck-Vaughn Company, Liz Anderson and Ellen Northcutt
GED: Science by Steck-Vaughn Company
GED: Science Exercise Workbook by Raintree Steck-Vaughn
GED: Social Studies by Steck-Vaughn
GED: Language Arts, Reading by Steck-Vaughn
GED: Language Arts, Writing by Steck-Vaughn
GED: Essay by Steck-Vaughn
Barron’s
Barron's GED 2008-2009 with CD-ROM by Murray Rockowitz, Ph.D., Samuel C. Brownstein, Max Peters, and Ira K. Wolf, Ph.D.
Pass Key to the GED by Murray Rockowitz, Ph.D., Samuel C. Brownstein, Max Peters, and Ira K. Wolf, Ph.D.
How to Prepare for the GED, Canadian Edition by Murray Rockowitz, Ph.D., Samuel C. Brownstein, Max Peters, and Ira K. Wolf, Ph.D.
Math Workbook for the GED by Johanna Holm
Essential Words for the GED by Sydney L. Langosch
GED Writing Workbook by Katherine S. Hogan
Peterson’s/Arco
Everything You Need to Score High on the GED: High School Equivalency Examination 15th ed. by Seymour Barasch, Ronald Kappraff, Robert Ganz, and Barbara Hull, and Kathy A. Zahler
Master the GED 2008 by Peterson’s
Master the GED - Language Arts, Reading by Peterson’s
Master the GED Science by Peterson's
Master the GED: Mathematics, 3rd Edition by Peterson's
GED Basics by Nancy Lawrence
Learning Express
GED Test Prep by LLC LearningExpress
Acing the GED Exam by LearningExpress
Research & Education Association
GED w/CD-ROM - The Best Test Prep for the GED: 7th Edition by S. Cameron, J. G. Emmons, Mel Friedman, L. Gregory, M. A. Kay, D. K. Klug, Gary Land, C. M. Mallek, Sandra A. Marona, and Brian Walsh
GED Mathematics: The Best Test Prep for the GED by Michael W. Lanstrum, Mel H. Friedman
GED Social Studies: The Best Test Prep for the GED by Lynn Elizabeth Marlowe
GED Language Arts, Writing: The Best Test Prep for the GED by Lynda Rich Spiegel and Anita Price Davis
GED Language Arts, Reading: The Best Test Prep for GED by Elizabeth L. Chesla and Anita Price Davis
GED Science: The Best Test Prep for the GED by Arthur R. Wagner, Jay Templin
Miscellaneous
Cracking the GED, 2009 Edition (College Test Prep) by Princeton Review
Kaplan GED 2007-2008 Edition by Caren Van Slyke
CliffsTestPrep GED by Jerry Bobrow, Ph.D.
Step-by-Step Guide to the GED by SparkNotes Editors from Barnes & Noble
The GED For Dummies by Murray Shukyn and Dale Shuttleworth, Ph.D.
