I used to use plain text files. Recently I started using Mendeley because it's so damn convenient and it has a very nice UI, but my problems with Mendeley are (1) comments are private and (2) it's proprietary.
http://www.researchblogging.org/ is a great website that aggregates more long-form posts about papers, but it has a really shitty UI and it doesn't do anything for short comments on random papers by random non-bloggers.