Monterey Bay Aquarium has a free app called "Seafood Watch" which helps you buy/order sustainable seafood. They had a booth at the Sunset Magazine Expo, which is how I found out about it.
I use to have a list that I got from my Environmental Studies class, but the app is better for several reasons.
One: Its harder to lose :)
Two: It has a much more intensive list than what is feasible to put on a little piece of paper and
Three: It is updated often.
If you eat sushi or buy fish to make at home, I really recommend downloading this app. I am not a super hard core "save the earth" kind of gal so this is one way for me to make a difference with a minimal amount of effort.
There is a lot more information on their website, go check it out!