As with compost you can pretty much use anything as a liquid fertiliser, there are also a few different ways to end up with your liquid gold. Here's just a few to get the thread underway and how I make mine, there is no waste that I can think of as the liquid is used to water plants and foliage and the solid leftovers can go in a wormery or on the compost.
Good for potassium so good for most plants, way I do mine is cut the skins up and put in a plastic bottle till around half full then fill with water. I then leave it for a couple of days then use to water the plants using a one part banana tea to four parts water, there is no waste as I cut the bottle open and use as a mini propagator and put the skins in the wormery. You could also put the skins on the compost or if your planting something, specially roses then dig your hole put the skins in then plant your plant.
This is more for the soil than the plant but benefits your plants in the long run, get a couple of pints of seaweed, chop up and put into 5 gallons of water then leave for a couple of weeks. Then just drench the soil, repeat every month in the summer if you can get hold of enough seaweed. One tip, if your going to use other liquid fertilisers then put this on first as this is for the soil, you want your plant fertilisers to stay near the plant roots and putting this on after would wash your plant fertiliser away from the roots.
Never used this so can't really tell you how good it is, however looking around it is used widely. This one's an easy one, you could either put a tablespoon into other liquid fertilisers depending on the ammount your using. Or just add a couple of tablespoons into a gallon of water then water your plants, what I've read it improves veg production and sizes.
So there's just a few which i'll add to in time or feel free to add your liquid fertilisers or your methods. Happy Growing, Neil.