I was staying in Foz de Iguaçu, and went to Ciudad del Este by bus; on my way I found a curious conveyance: the "mototaxi". It is a motorcycle with driver, that leads you to where you want to go; of course, you will have your own helmet.
Yo me estaba quedando en Foz de Iguaçu, y fui a Ciudad del Este en bus; en mi camino encontré un curioso medio de transporte: el "mototaxi". Se trata de una motocicleta con conductor, que te lleva a donde quieras ir; por supuesto, recibes tu propio casco.
Look for these white cars with green tops if you need a cab, although the main "sights" (you know, Cantonese Mafia, Nigerian hustlers, Arab arms dealers . . .) in Ciudad del Este are in a highly concentrated area near the Friendship Bridge, so it's unlikely you'll need one.
To get to Ciudad del Este from Foz do Iguacu in Brazil, go to Rua Almirante Barroso and catch the bus marked Ciudad del Este (it will also be marked "internacional"). It will cost 2.50 reias, which is less than 1 USD.
Your best bet for returning to Brazil is to hop on one of the white buses marked Linea Urbana Internacional. These buses will take you back to Brazil also for less than 1 USD.
You can take local buses from Puerto Iguassu or Foz do Iguaçu bus stations in both Brazil or Argentina - they usually have a sign 'Ciudad del Este.
I found the border crossing process a bit confusing. From the Argentine side, everyone has to get off and go through immigration before proceeding, you can leave your luggage because the bus waits for you.
Coming from Brazil you will have to let the bus driver know...some of them are nice of enough and they ask you if you plan to return and let you off and give you a ticket so you can catch the next bus.
Just remember to get an exit stamp in your passport if you plan to re-enter Argentina or Brazil.
If you are going to take the bus from Ciudad del Este to Foz the Iguazu, take it with more than enough time, at least 45 minutes before you thought.
When we returned we were waiting inside the bus almost one hour, and then when the bus started moving it took us more than half an hour to cross the bridge.
The time goes slowlier in Paraguay...
PD: Don't cross the bridge walking, is quite easy to be robbed, specially if you are a tourist.