This one has lots of pictures and breaks down the costumes into categories, like baby costumes, animals, group ideas (if your whole family wants to dress up with a theme), etc. Just looking at the pictures generated lots of ideas in my head.
This second site does not have pictures, but has descriptions of costume possibilities.
This last web site has pictures and detailed instructions. These costumes are specifically for children.
Of course there are many other sites with costume ideas. Have fun dressing up this Halloween!