It's called an Entrecard. It's a fun way to advertise on other people's blogs and to discover new blogs. How does it work? In a nutshell, you go to the Entrecard site, sign up and add their code to your blog.
Entrecard users will request to put their 125x125 pixel card on your blog. If you accept, it's automatically placed on your widget for 24 hours. (One ad at a time.) If you don't wish to advertise their blog on yours, you can decline the request. If you like the ads you come across, simply click and visit their blog.
You earn 'credits' for each ad you accept. You can also earn credits by clicking on the Entrecard ads on blogs you visit -- referred to "dropping" your card. These credits are how you pay for the ads you'd like to place on other people's blogs.
How do you know where to advertise? Clicking on "campaign" will allow you to browse Entrecards, click links to their blogs, and if you like what you see, you can request to advertise. Since I'm new and have few credits, I look for newer, inexpensive blogs in the art, fashion, or hobbies categories. I then check out their blog to see if I think it would be a good fit for my ad.
I avoid blogs of jewelry artists whose work are similar in style to mine. I also look at their blog's site traffic, and where they've placed the Entrecard widget. If it's too far down the page, I hit the 'back' button.
I've discovered some fun blogs via Entrecard. It's a fun marketing tool, one person I know received an Etsy sale from an Entrecard connection. It's no only for onling sellers. Non-commercial blogs are on Entrecard, too. It's for anyone who'd like to drive more traffic to their blog.
My Entrecard looks like this: