What could cause foamy urine, besides proteinuria?

Bile, air,medication: Bilirubin causing bilirubinuria can cause foamy urine, also fistula (abnormal connection) between bowel and urinary tract can cause pneumaturia, occasionally a medication like pyridium (phenazopyridine). Most common cause is proteinuria, which if present, should be worked up and treated.

Normal urine: Normal urine will produce foam depending on how the stream hits the already voided urine.