Mattercontrol: Though available for Linux, it's based on Mono, and starts up by explaining how the accompanying script will uninstall the version of Mono that the system has and install another from a different source. Not acceptable.
Netfabb: No longer has a free version, so I'm skipping it for now.
Cura: Starts out asking a bunch of questions before letting me download. No like.
Slic3r: Can integrate with Pronterface, so I don't have to relearn a UI. Reasonably easy to install and set up.
Slic3r does a Home at the start of the print, which takes quite a while for me. Then it lowers Z to a bit above the bed at 0, 0, heats up the nozzle for the first few layers (interesting), then moves to the center. I will need to move the clip off the corner, or it will run into it.
The print does look a lot nicer. I suppose I should have adjusted Skeinforge rather than Pronterface. This first print ended up being done at the edge of the tape, and the non-taped parts are curling up, so the tape is a good idea.
Sliced with Skeinforge. Notice how the layers are loose and seem to tumble down the side.
Sliced with Slic3r. Still skewing, but the layers themselves are much crisper.
The X and Y skew still happens. I turned the pots up half a turn. Also moved the problematic corner clip around the corner. Skew is still bad, possibly worse, there was a single-layer skew of an entire item width. There was audible skipping.
Turned the pots down to 1/8 turn below original setting. Skew is now less at a time, but still prominent.
Some new suggestions here: http://forums.reprap.org/read.php?151,202292:
Slow down the print, down to as low as 15 or even 10 mm/s.
Reduce max acceleration.
Adjust X steps (somehow, maybe, as a last resort).
Twisting the motor wires to reduce interference (maybe near the controller, but otherwise it's the same setup as for many successful Mendel90s).
Also maybe the gear is not solidly on the motor shaft.
If I am to ask for help, I should use the 20mm hollow cube.stl as my sample print.