The so called error function erf(x) is defined as
and it is well known that erf(∞)=1.
Are there any other known closed-form values of erf(x), except for erf(0) and erf(±∞)?
If the values aren’t listed on the Wolfram function page, I would be surprised if you found them anywhere else. The only listed closed form values are for 0, ±∞, and ±i∞. However, you can find various equivalent formulations, continued fractions, and the like on that page. A good reference, generally, for most well-known functions.