Unless of course its bean cooking liquid, then you can toss it away. I know some recipes use the liquid from beans but unless you live alone or in a frat house, I wouldn't recommend it. Bean cooking liquid will make you gassy like you never knew possible.
On the other hand, liquid from cooking pasta works nicely in soups and it is thicker than water which gives soups a nice bit of body. It was actually the cooking liquid from some faux chicken gluten that prompted me to post this though. The recipe I used said to cook the gluten about 50 minutes until the liquid had all been absorbed. 50 minutes came and went but the cooking liquid was still there. The gluten was good but what to do with all the broth?
Monday night I tossed it into the pressure cooker with some water, dried red lentils and a tablespoon of oil (to prevent foaming). Seven minutes later, I let off the steam and added in some chopped tomatoes. I served this over whole wheat couscous, which cooks in about 2 minutes. We had homemade coleslaw on the side.
Dinner took about a half an hour to get on the table and it was really delicious. So don't toss that cooking liquid, it may save you time another night. AT the very least toss it in the freezer to use another time but make sure you label the container.