One site that I've always enjoyed looking at is cruises.com. I recommend getting on their email list if you are thinking of vacationing soon.
They have discounted cruises, last-minute cruises, or short-term sales (like 48 hours or something). Some of their cruise prices are ridiculous. In fact, before I knew I was pregnant, my husband and I almost booked a 5-day trip to Mexico. It was only going to cost us about $230 each. NUTS! Have fun cruise shopping!