We used the Knot, and linked our BB&B, Amazon, and the Knot's honeymoon fund to it.
We are using Zola and I LOVE it. They are partnered with many stores so you can add their products right off of their website, but you can also register at other stores and add them onto your Zola registry!
We are registered at Target and Crate&Barrel, and both are linked into our Zola registry!
The Knot has a button "Add to The Knot" where you can add products from anywhere. Not sure how it works on the guest side, or if anyone will use it, but I like the feature since our stuff is from all over.