IOU can mean:

• a note and acknowledgment of personal debt (i.e. "I Owe Unto" or "I Owe U ^*") that is generally limited to casual usage between acquaintances.
• a cellular phone service provided by Vodafone New Zealand. If a customer sends the letters IOU by form of SMS (text messaging) to the number 468, he/she will get a Zealand dollar|NZ$2.00" target="_blank" >^* top-up to their pay as you go credit, which is repaid when the "debtor" next tops up his/her phone.
• an investor-owned utility (energy) company.
• a band that three members of Westlife were once in.

IOU