I compared some options in 2005: https://www.coli.uni-saarland.de/projects/irtg/contents/Coll... (Not that much changed, except that CiteULike was sunset.)

JabRef is a good option for a simple desktop GUI based program that is cross-platform, but I find myself writing "raw" BibTeX entries in Emacs or Sublime more and more often without any UI tools.