In addition to creating lists and nested sublists to organize your library, you can now add one or more searchable keyword Tags to any reference. To create a Tag, just click the + in the Tags field in the right panel, under the abstract.
Any keywords you create will be listed in the left menu panel, under the Tags section. All tags become links - clicking on a tag will take you to the list with all references containing the same tag. It's also quick and easy to search for any tag using the general Search bar at the top. To remove a keyword from a reference, just delete the hashtag from the Keywords field. A tag that no longer has associated references will be automatically removed from the left menu.