These! These are awesome! We had this one (Simple Human Slim Sink Caddy):
And we just upgraded to this one (the standard Simple Human Sink Caddy):
It's got a little round area on the left that holds my short soap-holding scrubby thing plus two sponges, but if you have a tall scrubby brush, you can pop that little black drain part out and it becomes a taller caddy to hold a longer brush.
It's great. The reason these are great is that they have suction cups (which we all know only sometimes work) AND an adjustable metal bracket that tucks nicely beneath your faucet and helps hold the thing on. Ours have never fallen off and both of 'em work great on the Airstream round sink.