[q3k]

A certain (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones.

[RachelF]

Don't forget BSD - all Apple operating systems are based on it.

[bawolff]

Arguably they aren't the "alternative" so much as a variant of the original thing that got open sourced.

[adastra22]

BSD stands for Berkeley Software* Distribution (of UNIX). It is a direct descent of UNIX itself, not an open source clone.

[fanf2]

Software, not standard

[adastra22]

Thank you.

[bawolff]

Indeed, that was my first thought. (Almost) Every mainstream best in class open source product was at one point the kind of crappy open source alternative to an established product.

[ThrowawayR2]

The original poster is asking about open source alternative SaaSes and Linux/BSD, gcc, Blender, Firefox, etc. is not a SaaS.