Apple II emulators are most broadly used on modern platforms, such as Sweet16 for Mac OS X or AppleWin for Windows. But the Apple II inspires a hacker's "can-do" spirit, and for the novelty if no other reason, the Apple II has been emulated in a variety of non-standard environments.
Video game consoles are likely targets for emulation, as unintuitive as that may seem. WiiApple was released for the Nintendo Wii back in 2009. It unfortunately hasn't been updated. It supports a USB keyboard as well as GameCube and Wii controllers as a substitute for a standard Apple II joystick. Similarly, Soul Captor for the now-defunct Sega Dreamcast was released in 2002. It too requires a physical keyboard — no iOS-style virtual keyboards here.
But perhaps most interesting is this Apple II emulator for the Fossil Wrist, a Palm OS-based PDA that was sold 2003–2005. This Dick Tracy watch featured a 160×160 B&W touchscreen and could run most Palm software. Using the program Appalm ][ (formerly PalmApple), the wristwatch could could be turned into a portable Apple II.
There are many more details about the Fossil Wrist on DonDula's blog post. And, of course, there are many other unusual emulators out there, which I cannot begin to attempt to catalog here. Suffice to say that, chances are, if it's a computer, you can upgrade it to an Apple II.