It can be any of the following:
1- It can be a transaction by mistake. But I doubt that someone would have been sending this little amount considering the transaction charges. So I doubt that.
2- It can be bounty money of some sort but I least expect that because it should have been not long ago that you have performed the bounty activity.
3- It might have been some faucet service that you abandon long ago and it decided to clear all wallets by depositing to their owner BTC addresses.
4- It can be some referral service to some service like a faucet when a person or more decided to follow your link posted somewhere and the referral amount generated. It can be one time only or can be first of the continuous income as well; depending on your referral activities.
My 2 Satoshis...