If you're like me, there is this presumption that under the bathroom sink is a wet, sopping mess. But I have yet to see this wet, sopping mess.
So why not carve out some storage space? You can re-purpose kitchen under-cabinet storage pieces or use generic wire storage racks.
One of the best--and expensive--solutions is this top-tier roll out under-sink unit from good old Bed Bath & Beyond. The top tier is inset so that it will clear the drain pipe.