How the hell was I able to receive BTCs via an address I do not own? A few minutes ago I received 0.00000001 BTC. I don't know who sent them or why.

But what makes it crazy is, this sum was sent to an address I do not own and which the client I'm running (the standard Bitcoin-QT, versione 0.8.5) had never heard of before. I know this for sure because I only created 2 (two) addresses using this client, and the receiving address of this transaction is not one of them.

How is this possible at all?
Is it a hash collision, or what else?

The address is 1Cz2AmLe8U52KR549d13JQkqsqkV64RchB and the transaction ID is b9591410ff030e40deb3eda9d2fe30a29642ede443f354bb5f4ed5bb55f0d36d.

According to this page, the address seems to have received money a little time before from 1HTawQNCS4axPXft6L6fkzrv5ya7TguqwD, which actually is one of my addresses; the last two transactions are me sending all my BTCs to a new address, just to be sure. This worked, so it looks like I'm actually the owner of that address, even if my client doesn't list it as one of mine.

What's exactly going on here?
As Murch says one possibility is that this address is in your keypool. Another is that this is a change address the client created. If you're willing to share the relevant transaction we may have more to say.

Owning an address means "being in unique possession of a corresponding private key". It's difficult to know if you're the only owner, but verifying you have a key is easy - ask bitcoin-qt to sign a message with this address.