Mendeley

Mendeley is my current preferred reference manager. Reasons why I like it: Automatic synchronization of references * Synchronization of up to 500 MB of attachments * Public profiles - see mine here * Shared collection - automatic sharing of collection of references * Use of sqlite for database - you can write your own scripts to read the bibliographic data * Automatic parsing of PDF files - not a big deal for me, since I import mostly from Google scholar anyways, but good for those old old papers * Extensions for Open Office, Word

Others to look at

Zotero

Another popular free reference manager built as a Firefox extension. Since I use Opera, this is less ideal for me.

Endnote

The old commercial reference manager that everybody liked before Mendeley.