I have been using Zotero for a while. I gave up on Del.icio.us a few years ago, and went to Zotero instead. I do not write to publish, so articles and websites that I want to remember are those that relate to a project I am working on or those that I want to share with coworkers. Citation generator capabilities aren’t of great importance to me right now.
For this Thing I did sign up and play around with CiteULike. It was a bit difficult and clunky to upload a citation from Academic OneFile. I was able to discover a few neat articles by searching topics I was interested in from within CiteULike, so it might be something I return to.
I used EndNote and RefWorks several years ago when I was working on my masters degree. I wish these types of tools had been around when I was working on my undergrad degree. It is so much better and more reliable than my previous note card method!