by Buback, published
This is a minimal Mendel X carriage with a large central opening. It is meant to be a drop in replacement for the standard Sells Mendel carriage. With a bit of work I'm sure it could work just fine on a Prusa Mendel.
Importantly, this design uses bearings, so you either need to salvage them from your current carriage or have 9 extras on hand.
This design has a number of key advantages: it is quick to print and assemble; it is light; ; it has a huge central opening to accommodate many hot end designs plus insulation and/or fan; No captive nuts; Easy access to extruder mounting points; it can 'clip' on and off of the x axis rails and belt.
Edit: I should mention that there are only the two compatible mounting holes, and that they are perpendicular to the regular x carriage. This is so you can mount a wades extruder sideways. There are other holes, as you can see in the renders, but they aren't in the same place as the regular carriage (at least i didn't design it to be, maybe i got lucky)
Also check out the the Y carriage redesign: http://www.thingiverse.com/thing:6641
Go here for instructions: http://reprap.org/wiki/OpenX
There are also some part variations on the bottom of the wiki page that some of you might be interested in.