Raspberry Pi

[Tommyand]

Build a NAS!

But if you want to do stuff like build mechanical arms and RC cars, I think a Arduino would be better for that.

You would probably be right. But did you ever wonder why there's an Arduino that also runs Linux?

On to ideas:
It is possible to setup a Minecraft server on a RasPi using Spigot, the awesomesause server that runs better than the regular minecraft server, or perhaps a server written in C++.

You could host a web server.

You could build a device that will play games for every console that can be emulated!

You could turn it into a Wi-Fi router.

You could do something awesome that nobody has thought of before!

That said, Good Luck!