According to the Zeo finale, it was Katherine Hilliard but more recently in Dino Thunder, there's not a clue who she is or even if he is.
Actually, in the episode "A Season to Remember" in the Zeo, it shows Tommy as an old grandpa telling a story to his grandson. At the end of the episode it shows Katherine Hillard as the grandmother and Tommy's wife. Also, originally in the episode "Forever Red" in Wild Force, it was supposed to show a cameo of Katherine Hillard at the end of the episode coming to pick up Tommy, but the idea was tossed because of budget costs. But since Disney kind of ruined the storyline by not showing if Kathernine was Tommy's wife or not, it is not very clear. Only in the Saban Era, you can assume that Katherine is his wife.