I am a HUGE fan of a "Do it yourself / Homemade" costume. I mean, that's half the fun of Halloween. ANYONE can go and BUY a costume, but the legendary ones are made from your very own hands...
Some people are more creative than others, so let me introduce you to a place to find some really good ideas and inspiration. "Coolest Homemade Costumes" came up with their new search engine this month called the "Costume Wizard".
It's an "In" to their database that basically helps you come up with the costume of your desire. Tell them who you are / what you're looking for, and they'll bring up all the entry's they've had that match those descriptions.
I have to say, some ARE better than others... and you will probably see my smiling face on there from my Jellyfish Costume fame... but I know for a fact it will get some of your creative juices flowing!
If you have a great Halloween costume you've created send it on over to them and help them bulk up that database!