But they make them more irregular still.

beau, beaux = male admirer(s)
bureau, bureaux = desk (a "false friend," = faux ami)
cheval, cheveau = horse(s) [edit 12/21: Shame on me. Attention wandering. "cheval, chevaux"]
cheveu, cheveux = hair(s)
eau, eaux = water(s)
gateau, gateaux = cake(s)

and even

l'oeil, les yeux = eye(s)

and I'm sure there are more I don't know