1. Your soul.
2. Alt coins well traded or not so much as long as it ain't dead.
3. Generally something worth more than 120 per cent.
I could give my soul lol
Physical items, like a computer, monitor, physical bitcoins or alike would also do.
Sadly, I do not accept souls as collateral, but I guess Lone Shark might be interested.
Edit: To add to this:
If you offer better collateral you will get a better rate.
Before you ask, no. Account do not justify. Not even your steam account with $10'000 worth of games. ID is not valid either.
Spotify, Paypal, Namecheap, Reddit, Poloniex, CoinBase, Localbitcoin or Pornhub are not valid either. They are accounts. Yes, all has been proposed before.