Site to find day of flight deals

Google flights/ITA Matrix. Set up alerts to multiple destinations.

For premium fare deals, FlyerTalk forum is consistently good.

There's an iOS app called Escape which will paint you a map of where you can go for under X dollars at the moment.

Literally day of? There's not a lot of places that do day of deals because the market is just not there for them to make it worth anyone's time or effort.

There's a lot of websites that you can put in your home airport and they will send you deals, but most of these are 45-90 days or further out.