Does anyone know where you can find last minute airfares that let you pick the time (within a few hours anyway) that you fly?

I am looking to fly from SFO/OAK to Dallas this weekend.

Many of the airlines have internet fares that are announced on Tuesday or Wednesday for the following weekend. The rules vary a little between airlines, but generally you leave either on Friday night after 7 PM or all day Saturday, then return anytime the following Monday or Tuesday. You do get to select the flight times (from what is available). The routes vary every week, so there's no predicting what will pop up. www.bestfares.com shows a comprehensive list of all the offerings, usually by early Wednesday afternoon. American announces their specials on Tuesday morning, so you may want to go to www.aa.com, click on internet specials, and see what's available.
Also try www.delta.com. The usual rate for these fares on the SFO/DFW route is about $180 + tax.

The real question should be "is any site worse than PCLN/Hotwire?"

The answer is no.

If you do your Internet search and don't find any low fares, you might find it worthwhile to fly Vanguard on that route. No FF miles, no first class, no MRTC, and you will be on an ancient 737-200 (I think they phased out the DC-3's last week ) - but if rock-bottom cheapness is a priority, it's worth a look.

your best bet is www.smarterliving.com. look under the "airfare guide."