Abstracts are available in BrowZine for all articles where PubMed provides abstracts as well, courtesy of the U.S. National Library of Medicine. In the web-based version of BrowZine, abstracts are shown directly below the article in the table of contents view. If the abstract is long, click the small down arrow to expand the view to show it all.
In the BrowZine apps for iOS and Android, the abstract will be shown on the loading screen after you have tapped an article to download.
Additional abstract sources are being researched to provide additional coverage in the future.