What does everyone think about the yongnuo speedlites?
They have a good range of models that could fit different budgets and they seem to have good reviews from strobists.
Apparently only yongnuo's most recent models include high speed sync... So if that is a concern your choices in their lineup may be limited to a couple at higher price point... Which may still come below Canon's price range but may not be by much, so I would choose Canon's own midrange slaves 430ex II or just the older 430ex for a few more bucks and have no surprises.
But for a 2nd or 3rd ETTL slave in portrait studio kinda work, I think yongnuo should be just fine.