1. Trash Bag Spider Webs
2. Pumpkin Lights
3. Card-boarded-up Door
4. Mummy door
5. A few tips...
If you or your roommate(s) don't own something you need, check with your neighbors. You'd be surprised what the rest of your friends have in the bottom of their desk. Check with offices on campus or businesses downtown for harder-to-find items. Plus, that's free.
Don't be afraid to hit up the trash room. First come, first serve. AND FREE.
Use what you already have. Re-purpose those free things you picked up at events and will never use or simply make something you already have a little more festive using nondescript party supplies in the colors of the holiday. Reduce clutter and spend $0 (aka it's free).
Have fun and get creative!