Foster found that persistence paid off with his 85th minute strike, as he had had two earlier headers cleared off the line.
A point was no more than Crewe deserved as they dominated the game in the second-half.
It was in sharp contrast to their performance in the first period when Walsall were dominant and deservedly took the lead through Jorge Leitao's fourth goal in five games.
But Walsall, who picked-up their first point of the year, could not hold on to the advantage he had given them.
The Saddlers had threatened to break the deadlock before Leitao struck, as they made a sparkling start.
Leitao had two shots saved by Clayton Ince while winger Darren Wrack also fired narrowly wide.
Walsall's pressure finally paid off when skipper Simon Osborn made strides down the right wing in the 26th minute.
The former Wolves' midfielder then whipped in a cross that flew straight to Leitao and the Portuguese marksman nonchalantly flicked the ball past Ince from six yards.
As is often the case, falling behind sparked Crewe into life and, after Ben Rix fired wide in the 31st minute, it needed a stunning save by Andy Petterson for the home side to retain their advantage.
Petterson, who is on a short-term contract to cover for the suspended James Walker, had been under fire after conceding nine goals in his first two appearances. But he made amends after 33 minutes when he somehow managed to tip a header from Dean Ashton over the bar following a right-wing cross from David Wright.
After a half-time roasting by manager Dario Gradi, the Railwaymen were a transformed side at the start of the second period.
Once again, Petterson rescued Walsall - saving a deflected 49th minute shot from Billy Jones.
Sixty seconds later, central defender Ian Roper was the Walsall hero when he cleared a header from Foster off the line.
Petterson then rescued Walsall for a third time after 62 minutes when he threw himself low to his right to beat away another powerful header from Foster.
But Petterson was badly at fault for Foster's equaliser. He was caught in no man's land when he raced out of his goal after a cross from Kenny Lunt and Foster took advantage to head home into the unguarded net.