One woman's junk is another's paradise. You may not love that sweater, but there's a woman out there who will consider it her new favorite thing. If it's extra incentive, donations are tax deductible—you'll be glad you did so come tax time!
Thrift stores, The Salvation Army, Dress For Success, Goodwill, and many other places will be really good to be the recipient of your good will (or disagreeable taste, whatever the case may be!). Charity Navigator's search feature can help you find charities with specific needs in your area.
A great way to donate with a spin is a service like Freecycle. You just post the item you are trying to unload, and someone who lives nearby who needs that item will come pick it up! This is also a great asset when you are cleaning house during a move.