I see these "Public Notes" on blockchain.info. What are they? How do they work? Can I make one? Here's a random example: http://blockchain.info/address/1DkyBEKt5S2GDtv7aQw6rQepAvnsRyHoYM Some of the transactions on that address have "Public Notes" before them.
Public notes are simply attached to addresses/transactions in the blockchain.info database, not in the blockchain itself, it's more of a convenience service offered by this particular website than a protocol feature.
And yes, you can make them on blockchain.info, you apparently do not need to own an address to tag it (some of my addresses got tagged without me doing it). The same is probably (I haven't checked) true for transactions.