It started as MacAmp, then was renamed to Macast in '99. That and SoundJam MP were the two big ones for OS 7.6 and 8.x. I used MacAmp/Macast on my System 7.5.5 setup, too.
I'd say ClarisWorks would be another big one to have. Not sure which version corresponds to which OS, but most Mac folks had it right up until AppleWorks came along. My dad always had FileMaker, too.
When was FreePPP? That was how I connected to the internet for a spell in the 90s. Speaking of internet, you've got to include Hotline on System 7.5 through to OS 9.x (and maybe even up to 10.2). I miss the heady days of Hotline.
Glider PRO and Escape Velocity on OS 9, maybe OS 8 too. Glider 3, StuntCopter, Dark Castle, The Fool's Errand (although I spent more time with At The Carnival) on System 6. Actually, a bunch of apps and games listed
here would have been on a typical System 6 Macintosh.
Marathon on OS 8? I can't remember. I spent a lot of time with Super Maze Wars, Spectre, Spelunx, and Eric's Ultimate Solitaire in the System 7.1-7.5.5 days. I'm not sure which of the crapload of shareware games I played were common.
If I think about it for a while, I can probably figure out exactly what software I had on System 6 and System 7.5.5 (the two main ones I used growing up).