After you've done some scrubbing with the rough side of a sponge, there's a good chance that nasty food gunk will be stuck in there. A soak in some salt water will loosen it up and make your sponge look almost new.
Sponges seem to absorb more than water; they take in food bits and burnt grease too. This video from the HouseholdHacker YouTube channel -- around the 1:20 mark -- shows you that a good soak in some salt water can help get that nasty stuff out so it doesn't start to stink up your sponge. Add 1/4 cup of salt to about two cups of water, toss the sponge in and let it sit overnight. All that gunk should be gone or easily removable in the morning. Make sure you have another sponge around while you soak it, just in case. Sponges aren't terribly expensive to replace, but why waste the money when you can revitalise the ones you have?
7 Salt Life Hacks You Should Know [YouTube]