I'd say Mozilla/Firefox somewhat counts. They acquired Netscape around 1998, and although they eventually ran Netscape itself into the ground AOL-style, they allowed the devs to work on Mozilla for quite some time. Long enough so they could create what would become the foundation of Firefox (Gecko and XUL, most notably).

Mirabilis, with ICQ? I don't remember when it peaked but it certainly was huge. I was using it heavily around 2000-2003.