Garachu is a Chrome extension that lets you write down what you're grateful for each day. You can even let it do the work for you by simply listing the three things you're grateful for on a daily basis. The extension then gives you an option to let it remind you of these things on each open tab. So you don't even have to write them down as it will handle the work for you.

You will be reminded of the things you are grateful for each day in your new tab.