JabRef allows you to create custom export filters and style files for LibreOffice. This functionality and the installation procedure are described in the help file on Custom export filters. Some users have created export filters that can be useful to many others.
BibSync is a tool to synchronize your paper library with a BibTeX file which might be most useful for Physicists and Mathematicians since it supports synchronization with DOI and arXiv.
Bibtex4Word is an add-in for Microsoft Word that allows the citation of references and the insertion of a bibliography into your document using your choice of formatting style. It is lightweight, transparent and does not mess up your documents.
Library is an Open Source Android application to view BibTeX files in order to open (and annotate) linked PDFs. It also supports opening DOIs as well as sharing entries. Furthermore it is possible to filter entries and sort them by date, journal, author and title. There are no editing capabilities though.
Feinerleiser is a tool for improving the JabRef-LibreOffice integration when writing for the humanities. This tool can be run to finalize a document, providing citation features that are not supported by JabRef itself.
This site offers to generate .gitignore files using common patterns for applications. For instance, you can use the keywords JabRef, Windows, Linux, macos, latex to generate a .gitignore for your daily tex work.