I have been looking at man made islands for a while. Indy Island in Indiapolis was created by Andrea Zittel a couple of years ago, it floats in a lake near Indiapolis Museum of Art. They offer artist residencies on the island for 6 six weeks each year. I would love to do a residency there.
Sealand has a bizarre and long winded story behind it's existence... but in short it looks like an oil rig but it's a country off the Felixstowe coast complete with a monarchy, national anthem and currency.
No Man's Land Fort
Spit Fort Island even has its own lighthouse