I didn't really have a single "Aha" moment, but I remember borrowing
some books on Unix from the library and realizing it was more powerful
than anything else I'd used (up until then, I'd only really used
DOS/Windows, classic Mac OS, and Apple II systems; as you can probably
tell, I'm quite a bit younger than many other people on this list).
Shortly afterwards, I installed Linux (initially Mandrake 8.2, but I
replaced it with Debian 3.0 shortly thereafter; I still have my
original Debian 3.0 install around as a VM that I use from time to
time) and never really looked back. I did keep a Windows dual boot
around for a while but that eventually went away (although I still do
have Windows VMs around). Soon after that, I decided I was going to
put together my own Unix-like OS; initially I was going to put
together a NeXTStep/OS X-like Linux distribution, but then later
decided I was going to write a QNX-like microkernel-based OS instead.
I still don't have anything that is actually useful at the moment,
although now I am making a bit better progress than in the past (I
changed my mind on several parts of the design and was quite busy with
other projects for a while).