The free lookalike / work-alike / supports the MS file formats (*) office suites that I have used for years are Open Office and Libre Office. They are plenty good enough most of the time, but despite supporting the file formats, these on occasion won’t do everything 1:1 identical to the result you would obtain from, say, the real Excel or Word. And, once in awhile, that can be a problem. If Ability added anything significant to the mix, I might be interested in having another alternative. However, some of the reviews and comments lead me to think that this is not the case.
(* which have themselves changed, over the years)