I know it’s not en vogue right now, but I started using @Evernote in August of 2008. I’m still using it today.
Trying to think of other software I’ve used a similar length of time and the only thing that comes to mind is OmniFocus. I first purchased it with my first iPhone which was the iPhone 3G. This quickly led me to buying it for my Mac and a few years later on my iPad.
What software are you using from 10+ years ago?