Google scholar has started a facility for us to build libraries (see announcement) of articles that we are interested in. I am not sure how useful this will be since I have to maintain a library in my citation manager in any case (I use zotero now and have used Refworks in the past).
The Library allows us to save articles while we are searching for them and it can also give us recommendations based on the library (which Zotero cannot do). That could be a useful feature temporarily as we search.
The coolest feature so far is that it can create a one-click library for you based on all the articles that you have cited in your articles – and thus can build a library with hundreds of citations in seconds. But for this to happen, you should first have a scholar profile (not a bad idea to have one, in any case).
Overall, without citation support, this is bound to be a mildly useful feature to have around.