Most dive travel vendors highlight their specials but you have to go looking for them one-by-one. But I think I know what you mean, a wholesale site that lists destinations and places with last-minute trips and hotel-dive options; that would be a great reasource.
Are you interested in liveaboard diving? Sometimes they have open slots for scheduled departures and you can get a good deal. Liveaboard.com is good site to watch. They'd rather sail with a full boat.
You didn't say what part of the world you are most interested in diving, but right now the Caribbean Explorer II has good deals for December dive trips to Saba and St. Kitts, see the link below. They are asking $225 to $300 per night per person and that includes food, accommodations, and lots of diving - and it is very nice Caribbean diving. Airfare and tips would be extra.
Do you travel with a buddy? If not the single supplements can be brutal at resorts and on liveaboards. Most liveaboards will sell single berths and pair you with a same gender buddy, so that can be a good deal if you don't mind that. And if they don't sell out the boat, they may let you have the cabin without a buddy and without the single supplement charge. It's easy diving too, you set up your gear once at the beginning of the trip and then they fill your tank in place between dives.
Another nice thing about liveaboards is that they don't need a lot of advance planning.
@drrich2 calls it "turn key" diving and that's a good comparison. You just have to show up on time with your gear and C Card and everything is already arranged for you. Plus it is a lot of diving, and often very good diving.
Caribbean Explorer II
