The outdoor store I buy for sells cases of the Sea to Summit Kitchen Sinks. The come in 5, 10 and 20 liter versions. Really a slick product that I use on every outing. For that matter, I'm a massive fan of Sea to Summit's entire line.
Or, another really slick sink solution is the Wash Station from Byer of Maine. It's a free standing wash basin with three compartments for wash, rinse and dry. I have yet to use mine, but when we were at the Outdoor Retailer Show, Jonathan Hansen and I both agreed it was one of the more interesting items at the show.
http://www.byerofmaine.com/trilite-wash.htm