Sun Microsystems happened to be across the road from us. My mind was blown the first time I saw the value of the invoices for servers and Solaris licenses that we bought both for ourselves and on behalf of our customers. That's where a lot of those dotcom-era "investment" dollars ended up - at Sun.
One day we needed a router + firewall for some internal service. One of the Unix sysadmins in the team grabbed a spare i386 desktop PC, stuck a 2nd NIC in it, installed Slackware Linux and configured ipchains. Job done: no budget, no managerial approval, no licenses, nothing. I couldn't believe it.
I asked him about Linux and after learning more came to the conclusion that it could basically do most things that Solaris could do but was 1) free and 2) ran on cheap, commodity hardware.
That was the writing on the wall for me. I taught myself Linux and pretty soon had my first bona-fide Linux Sysadmin job. Linux went on to become the OS that runs the world and I've never struggled to find relatively interesting, well-paid work since then.