There are some of the called Stumbleupon alternatives but the one I noticed no one discussed about it. There is a great Stumbleupon alternative that I use. I like Stumbleupon very much for finding new information I am using it from sometime. As we know Stumbleupon is a very large so no other alternative site as large as it is.
The one good alternative of Stumbleupon I found is Infoaxe, I think it is a real alternative. To use you also not need a toolbar, however you can install Firefox add-on. Infoaxe is not very similar to Stumbleupon but you can browse the sites in same way.
Stumbling button for Infoaxe is known as Flip button.