An optimized key component for RepRap Mendel - the frame corner bracket. This version is less prone to warping, easier to print, uses minimal material while providing excellent mechanical properties. This final version comes after several weeks of testing various different designs, although there are two key designs for comparison:

[A] 'Low warp, shelled Frame-Vertex' by MarcusWolschon
http://www.thingiverse.com/thing:2413 (frame-vertex 6off.stl)

• Elegant design, uses compliance to minimize warping. Very minimal use of materials (~14cc).
• Poor mechanical properties. Not sufficiently stiff or strong. Would require infilling with epoxy or resin which is expensive. Difficult to print due to very thin walls.

[B] 'Mendel frame vertex thin shelled' by camiel
http://www.thingiverse.com/thing:4173 (001010 frame vertex vc2.stl)

• Structurally very stable. Fairly minimal use of materials (~19cc).
• Excessive warping. Cannot be printed in ABS without a heated bed. Some thin walls tricky to print.

The volume of the first 2mm of a print, is a good indicator for warp. (See image for comparison of design footprints). Design [A] uses complex geometry to reduce warp, but has a moderate footprint due to the solid base (3.0cc). Design [B] is worse than the original Mendel bracket with the greatest footprint (4.3cc). By eliminating a solid base and using shelled geometry, this design has a reduced footprint (2.5cc) and substantially less warping.

Depending on your setup, this only uses ~19cc of plastic and takes under 1hr 15mins to print. When printed in ABS with a raft (no heated bed), the warp is under 2mm at the extremes (black ABS in photos). In PLA there is essentially no warp (red PLA in photo). It is really important to use a squished raft or first layer to control warping.

The part has been aligned for printing on a Makerbot. It is rotated 45° to: minimize the footprint size (- useful if you are struggling to get a flat print bed); maximize the area of the raft printed (- helps further with warping). However, to play nicely with Skeinforge you may want to check the Infill angle doesn't clash (change from 45° to 90° as per screenshot).

Have fun!

Note: This is just a dirty-rude Meshlab hack of Rhys' original:
http://www.thingiverse.com/thing:3904
A future cleaner version may be released, but will not substantially change the design.