Many dime novels and nickel weeklies are available online; here are some places to find them.
American Women's Dime Novel Project: This site, hosted by George Mason University, focuses on dime novels written for women and features background material (including primary sources and detailed bibliographies), cover galleries and a few full text links.
BGSU - Nickel Weeklies: This collection includes digital copies of hundreds of nickel weeklies.
Dime Novel Castle: This commercial firm sells electronic editions of dime novels and has a few free samples available.
Dime Novels at Northern Illinois University: This growing collection contains many high-quality dime novel scans.
The Edward T. LeBlanc Memorial Dime Novel Bibliography: This bibliography, currently a work in progress, aspires to put online the extensive scholarship of former Dime Novel Round-Up editor Edward T. LeBlanc and link individual entries to online full text where available.
Northern Illinois University: Beadle and Adams Dime Novel Digitization Project: Although somewhat difficult to navigate, this site has useful resources including a detailed history of dime novel pioneers Beadle & Adams and full text for the very first dime novel.
Project Gutenberg: This collection of free online books contains quite a few dime novels, but there is no easy way to find them all. Try searches for popular dime novel pseudonyms like Nicholas Carter or Burt L. Standish.
Stanford: Dime Novels and Penny Dreadfuls: This site offers general background material and a few full-text editions reformatted for online reading as well as quite a few PDF issues of Secret Service.
University of Minnesota: Hess Collection: The Hess Collection features a variety of early popular works, and their website includes several digital editions of dime novels.
University of Southern Florida Digital Collections: Dime Novel Collection: This online collection provides representative issues from a variety of series, viewable online and downloadable as PDFs.
These libraries have pages devoted to dime novels but do not offer online full text as of this writing.
Library of Congress Dime Novel Collection: Although the Library of Congress has a large collection of dime novels, their website features only a brief description and a small exhibit with cover images.
New York Public Library - "Impossible Trash": The New York Public Library is another institution with a huge physical dime novel collection; this small online exhibit highlights some of their treasures.
Northern Illinois University: Edward T. LeBlanc Collection: This extensive collection complements NIU's Beadle & Adams project, but unlike that collection, it has not been digitized.
Syracuse University: Street & Smith Preservation and Access Project: Syracuse has a large collection relating to dime novel publisher Street & Smith; although no full text is online, this site offers images of many covers.
These pages don't provide direct access to dime novels, but they do provide useful information about them.
Dime Novel Round-Up: The journal of dime novel collecting and scholarship, Dime Novel Round-Up is well worth a look if you want to learn more about the field. This page offers an index of articles and subscription information.
Wikipedia: Dime Novel: While Wikipedia is not always the most reliable source for information, it does provide many additional links on the subject.
If you are interested in some of the dime novels' precursors and successors, these resources provide a variety of examples.
Emory Libraries: Yellowbacks: Emory's library has produced PDFs from more than 1,000 Yellowbacks, an early relative of the dime novel. To access the files, it is necessary to navigate their online catalog.
The Pulp Magazines Project: This site features PDFs of a wide variety of pulp magazines, the immediate successors to the dime novels.
University of Minnesota: Beadle's Weekly: This archive contains digitized issues of the Beadle's Weekly story paper.