Diigo <- check it out!
I own a lot of forms of transportation... I have both rollerskates and rollerblades. While they are both very similar, they both have 4 wheels, they both fit my feet nicely and snug up past my ankles... They fundamentally differ. For diigo- while I can work collaborative by sharing bookmarks, I can just as easily make a shareable Google Doc. For making annotations, I habitually use my Surface tablet with it's awesome pen. I know it has some additional features, I just never found a use for them.