Epbot (I highly recommend that blog if you're not already reading it). It's all about creating costumes based on real women. Scientists, artists, explorers, queens - they've got it all. They don't sell anything, they just offer resource guides and they are absolutely wonderful. For example, how about dressing like Jane Austen or Athena or Baba Yaga? These are just a few selections from costumes on their site. They provide reference photos (and digitial illustrations of the costume in most cases) and a complete list of necessary components for the costume. The thorough breakdown offers suggestions on where to get each item and provides links to purchase them online. Yeah, it's fabulous.
It gets better and easier, too. None of the costumes they post require sewing. They are built around everyday items you may already have in your closet or pieces that are available for purchase.
Kickstarter right now to fund adding more costumes).
And yes, they have a last minute ideas section. Sure, Halloween is tomorrow and that doesn't leave much time but if you gather supplies tonight and stay up a little late... it could work out. Good luck if you're scrambling! Actually, I'm curious - what are you plans for costumes this year?