I've added a series of new scalers to MAME4ALL for the upcoming v1.1 release.
* Scale Horizontal.
Basically the same as notaz's scaler, whcih is also used in MAME4ALL Wiz. Lets you stretch and shrink the horizontal axis only to fill the entire screen width. Because of the way it's coded, it may not work for some games which don't scale evenly, but the results are nice and it's fast. Useful for high horizontal resolution games (like R-Type, Final Fight and Gauntlet, etc) and lower resolution games (like Pole Position, etc).
* Scale Best Fit.
A new scaler that lets you shrink both horizontal and vertical axis and maintaining the correct aspect ratio. It also does pixel merging so you the display looks quite good (IMHO, though others may disagree).
This scaler produces high quality results at the cost of some speed. It allows you to see the entire screen for high res games (like Super Sprint, Rampage, Tapper, etc). It also lets you see the entire screen for portrait games (like 1943, Twin Cobra, etc) without having to rotate the handheld 90 degrees around.
* Scale Half Size.
A new scaler that renders every other column and line. It's quite fast and can be used as a substitute to Scale Best Fit. It's really been designed for high res games (like Rampage and Tapper). As it just drops pixels it doesn't look as nice as Scale Best Fit, but it's quite a bit quicker.