If you're flexible on the exact dates of your last-minute trip, check out Yapta. It's an online price tracker that's known for tracking flights, then sending alerts when the price drops. It's useful if you're looking to get a refund on your ticket if the price drops after your purchase.
There's another feature that last-minute travelers might also find useful. It has a search function that lets you choose the option of being flexible with dates, then churns out a neat graph that shows you what the prices would be if you choose certain dates.
It makes it easier for users to better figure out which days to travel on since it's all laid out for them to view.