Background: I came from a Treo 650, which I had a lot of open source software for, but was buggy and slow. I played movies on it, read ebooks on it, sshed, browsed the web, played NES and GB games, kept passwords, kept track of my mileage, etc; most everything you can do with an N900, but slower and not nearly as stable. Still, I've been very WOW'ed by:

• Streaming Internet radio around half the world and playing it in my car, without even an audio patch cable.
• Watching YouTube videos
• Having multiple web pages open and being able to switch between them quickly.
• Opening up a terminal and typing "cat /proc/cpuinfo" and "uname -a" and "perl -e ...", etc, etc.
• Finding equivalents to almost all the apps I used on my Treo already available for free.
• Keeping a copy of my favorite movies, music, ebooks AND the entirety of my local big band's sheet music (approx. 12k pages, approx. 300 pieces with approx 18 parts each piece) on one device (the storage on this device is incredible).
• Running Final Fantasy VII in not one, but two PlayStation emulators.

That may not seem like much, but keep in mind my standards are pretty high. All of the things I listed the Treo as doing, the N900 does and does better, so you could count those as "WOW" moments as well.

EDIT: 2010-04-07:
A few more that spring to mind:

• Exporting my N900 desktop over VNC to my desktop computer.
• Mounting NFS off my file server at home to play all the myriad media I have on it.
• Plugging in my N900 to my TV and seeing fullscreen playback work beautifully,