When making strained yogurt, what can you do with the leftover whey?

Use it for making bread or roti dough..

Kind of odd sounding answer here, but I read this in a landscaping magazine. Some people who have rock retaining walls use yogurt to spread on the rocks where they would like a little moss to grow. Somehow attracts the moss spores a little quicker and lets you choose where you want the moss instead of it choosing. I would still google it though.

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lapidia, a food52 cook, has it covered in an entry on her blog:
http://lapadia.wordpress.com/2010/09/19/curds-whey-uses-for-whey/

I wouldn't hesitate to water my plants with it, and traditionally (I believe) it's mixed in animal feed.

Great for making lacto-fermented pickles. You need less salt, which I have found makes a much tastier pickle. Here's a good recipe: http://www.food52.com/recipes/5038_lactofermented_pickles_with_garlic_scapes Adapt this by using one tablespoon of sea salt and 1/4 cup whey. Have fun! ;o)

I freeze whey for later,or use it as the liquid when I make bread or you can feed it to the dog or chickens!