SQLite is so pervasive that I'm pretty sure everyone uses it at some point. It's in client applications, it's used by yum (the package manager found on CentOS/RHEL/Scientific Linux systems), it's part of many web applications, it's part of many spam filtering systems, it's part of Android (maybe iOS, too?), it's basically impossible to avoid it if you're a nerd.
Also, it's awesome.