First-half goals from Daniele De Rossi and John Arne Riise had Roma on top while Matteo Brighi briefly restored their two-goal advantage after Balotelli had side-footed his first goal five minutes into the second.
Balotelli, in the starting line-up as Zlatan Ibrahimovic was on the bench with a muscle problem, narrowed the gap again with a penalty in the 63rd and substitute Crespo, making a rare appearance, completed the comeback with a header in the 79th.
"A point is gold from a match like this," Inter coach Jose Mourinho told Sky television. "The match seemed lost at 3-1. My team deserves this point, not for the quality of play. Their character made the difference."
After being knocked out of the UEFA Cup, AC Milan's crisis deepened on Sunday with a 2-1 defeat at Sampdoria.
Milan protested furiously about Cassano's opener, arguing keeper Christian Abbiati had stopped the ball before it crossed the line, but TV replays seemed to show the referee made the right decision.
Ancelotti had already called an end to Milan's title hopes, saying a top three finish for direct entry into next season's Champions League was the target.
"We are conceding goals too easily," Ancelotti said. "We found ourselves 2-0 down even though we had played more or less as well as Sampdoria.
"It is a delicate moment that must pass and we must make it pass with the least damage possible."
Giuseppe Mascara scored the goal of the day with a remarkable hoof from just beyond half way for the third goal in Catania's 4-0 demolition job at Sicilian rivals Palermo, who had Mark Bresciano sent off early on.
