The same happend to me: 2 Raspberry Pi 1st generation, 1 Raspberry Pi 3rd generarion, 1 Raspberry Pi Zero and I wonder if I have a 2nd generation Pi, as well. All in the box...
I set them up for XBMC/Kodi (with the 3rd generation actually OKish), I tried them as a cheap computer (too slow) and I have actually one with LCD touchscreen setup to work with an RTL dongle as a spectrum analyser (GUI sucks, but I must not complain, as it is programmed in Python and so the source code could be improved).
After each application setup and running, I never touched them again. Oh, the Zero has the Emulator image, which is pretty neat (can't remember the name).
With Arduino it was the same: I got all these Chinese sensors, motors, etc. I even got the GSM900 module. Tried it all once and then never used it again.
I think the reason is actually quite surprising: if you want to build something substantial with these things, you end up spending more money than buying an off-the-shelf product that does the same (and probably better).
Example:
To run Kodi, you can get an Android based settop box for around 80 Euro, which has remote and runs Kodi fine. This wouldn't even be the cheapest and most basic one.
Take a PI and you need: a power source, HDMI cable, remote, SD card and box. All this sums up to the same (or more) as the settop box, but is probably less robust for continuous use.
For a gaming device, take a 100 Euro tablet and install the same emulators. They will probably run faster, plus you get a mobile platform with screen and battery, that can surf the net and run modern games, too.
I would find it more attractive to learn how to program an Atmel or PIC and build a board myself.
But this doesn't mean the Raspberry Pi isn't a marvelous device. It is! But to really make good use of it, you need a funky and/or bizarre idea. I daily check their blog and it is amazing what projects show up!
Cheers,
Vitor