I heard about this cafe by a photographer friend. It is a small narrow traditional cafe at the centre of Piraeus, where every Sunday, a singer comes along to cheer up the customers.
The cafe is located very near the train station of Piraeus and along the railway lines. There, every Sunday a flea market takes place. Many vendors sell really cheap used stuff from clothes to cell phones and the place is rather crowded. This is not a flea market for tourists but mostly for lowest income people.
You might pass at the front of the cafe and not even pay attention, so make sure you keep an eye for it. It is highly probable that when you see it you might not want to get inside, but I didn't regret my choice as I had a really nice time.
The singer, Theodoros, or Theo as everyone calls him, comes along probably at around noon carrying a small speaker - amplifier and a bin bag with microphones. As soon as he fixes everything that will be wrong with the system he starts singing. He's got his usb flash containing karaoke songs, that uses with his amplifier so that he can sing.
People have been waiting for him so they cheer up pretty soon. The singer is happy to offer his microphone to the customers to sing as well.
Soon dancing starts and everyone sings along. The place is crowded, probably due to the flea market. It's very likely that you won't be able to find a free table, but you can wait for one. Everyone is drinking his poison and there is also food you can order.
Many of the customers are regulars, so they know Theo pretty well. After a song, there is an applause and everyone shouting "Bravo Theo!" and they ask for specific songs. It is a bit difficult for Theo to find the right track, as everything is in USB, but after a few songs he will eventually sing the one you want.