My kitties all have runny eyes at the moment because of conjunctivitis.
Take into account that if it's yellow-greeny icky, eyes are red and eyelids pale or inflamed there's a good chance it is infection of either a viral or bacterial sort.
Try to rinse runny eyes with saline solution (1 cup luke warm water, one teaspoon of salt, drip into corners of eye) and wipe with cottonwool. If the eyes stay runny, the eye looks inflamed or in any way funny and this doesn't clear up within 2-3 days, have it looked at.
My kitties are on stuff called sulphacetamide which is a human eye ointment used for mild infections, ask your vet if he knows anything similar you can use. Conjunctivitis is basically kitty pink eye, and very contagious. Keep those eyes as clean as possible, and wash your hands after each handling of your cat (before you rub your own eyes or that of another pet) since you and all your other pets can get it too.
My friend who has a cat shelter has a kitty who has had antibiotic resistand conjunctivitis for 2 months and he's not dead yet, so don't freak out too much.
If it is just a clear discharge and your kitty isn't in any discomfort, it might just be a case of kitty flu or ,as some other posts mentioned, allergies. The main thing is to watch for changes in behaviour, appetite and temprature.