Considering a Chromebook? Chrome OS has its first worthwhile video game emulator: GPemu.
This is a multi-system emulator supporting Game Boy, Game Boy Advance, NES, and SNES. It works offline, has save states (right-click screen for access), and can load zipped ROMs. Emulation quality is pretty good with only a bit of scratchy audio.
The drawbacks: NES emulation is poor; it only displays blue and red colors. You need to manually enlarge the window to blow it up to a good size. There's no gamepad support, but that's not the emulator's fault because Chromebooks don't support them in general (yet?).
In any case, GPemu's a good start for video game emulation on Chromebooks. Hopefully others will continue the effort and bring more to Chrome OS such as Genesis emulation.
Check it out:https://chrome.google.com/webstore/detail/gpemu/jhficiigpnhhaojldmanflihieepanbb