Take It To Charity:
Local thrift stores like Goodwill or Salvation Army will usually have drop off points where you can take items. Local charities will sometimes even pick up donations, so check around. Specific items like toys might be a great addition at a church, school, or day care. Magazines and books might be of use at a local clinic or doctor's office.
Give It Away:
Even if you're not giving it away to charity, just giving your unwanted stuff away to someone else who wants it and will use it is a great option. Try Freecycle. Or for an even more simple give away, just place items on the curb in a box marked "free".
For nicer items that you still think could have a good amount of cash value, consider selling at an online auction site, or listing in an internet classified ad.
Trash It/Recycle It:
If it's not worth giving away or selling...? Well, you may need to consider putting your "treasure" in the trash or the recycle bin. Let it go, and live happy in your clutter-free home.