One year I decided to dress as a nerd for Hallowe'en. I took an old pair of glasses with large lenses, "repaired" it with some masking tape, and combined it with a plaid dress shirt with a pocket protector and courduroy pants that were too short.
Nobody at work realized I was wearing a costume.
The best Hallowe'en work costume I saw was someone who came dressed as a Christmas tree. Complete with Christmas lights. She had to make sure to stand near an electrical outlet so she could plug herself in.
I wish I had a picture: One of my friends spotted somebody at a work Hallowe'en party dressed as Bill Gates. But not the Bill Gates of today. He was dressed as Bill Gates from the arrest photo.