Is it only Catholicism that says grace on food? I've heard it referred to as saying grace when you say a prayer to bless the food. Which denominations do this? Is this only a Catholic thing? Where did it come from?
Christians of all denominations (Eastern Orthodox, Roman Catholic, Protestant) may all say grace before meals.  Some do and some don't consistently or not.

The origin of the practice is from Scripture.


  Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
  Speaking lies in hypocrisy; having their conscience seared with a hot iron;Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth.For every creature of God is good, and nothing to be refused, if it be received with thanksgiving:For it is sanctified by the word of God and prayer.
  
  1 Timothy 4:1-5


The word for thanksgiving is eucharist, which is a priestly word, denoting the sanctifying (setting apart) of the food to be received.