If you'd have told me 25 years ago that I'd be playing 3D Monster Maze on the bus on a device powered by batteries, I'd have said you were mad... Though if you'd have told me in the same breath we would have flying cars by now I would have believed you. Raymond Baxter has a lot to answer for.
So the upshot if this temporal rambling is DS81 which is an emulation of the Sinclair ZX81. The initial release (much like the original designers envisaged) laughed at the idea of high-resolution graphics on the ZX81, but since Version 1.2 DS81 has support for some high-resolution games.
You can download a copy of DS81 here. There is also available a version without FAT support here if the file enabled version causes problems. The latest development sources can be downloaded here.
Before you get too excited it should be noted that it is still early days for high-resolution support. Forty Niner by Cosmic Cockerel for an instance causes intermittent lock-ups. And Manic Miner won't run at all as it switches between hi-res and text in the same frame. This confuses the poor emulator no end. I wish it didn't -- it's a stunning bit of coding.
DS81 also includes a few games to try out. Note that I did not write these games, or own the copyright to them. They are included in good faith (as far as I know the authors do not mind this work being distributed), and if any of them want their work removing I'll gladly oblige:
ZX81 ROM © 1981 Nine Tiles Networks Ltd
Mazogs © 1981 Don Priestley
City Patrol © 1982 Don Priestley
Sabotage © 1982 Don Priestley
3D Monster Maze © 1983 Malcolm E. Evans
DS81 also supports the loading of external files from a FAT formatted device. This is managed through the Dynamically Linked Device Interface (DLDI).
The included text documents with the instructions and recent changes can be viewed here.