You can markup any web page just as their catchy name states. Rather than just send a link, why not make a mark next to the cool stuff you’ve read or write a big fat note (with their markup marker-like pen tool). This elegant web-app is terrific if you like to share ideas at the same time you share a link.
You can learn more about Markup.IO here or follow them on Twitter. Please note that it is a .IO domain suffix and not .com.