I was at a electronics shop today and they were having a sale of pretty much everything, so I had a look around. On one of the bargain tables they had a few Wii-mote Laser Sword (copyright ftw) attachments. They sounded pointless and pretty tacky, but they were $10 each so I grabbed one.
It is actually pretty sweet. It lights up the way a lightsaber should and it looks fairly decent. The one downside is that it is kinda short, but it's cool none the less.
What random, cheap or purposeless crap have you bought?