Acid was pretty ground-breaking at the time it came out, I think a lot of beat-matching features in modern DAWs was inspired by what Acid did. It was made buy Sonic Foundry who then sold it to Sony - http://www.sonycreativesoftware.com/acidsoftware

I have an unusual amount of knowledge about Acid since a friend of mine originally created it (he left Sonic Foundry to work on audio software at Apple the last time I talked to him) and I used it to produce two albums back in the day.