With Valentine's Day just around the corner, I'm uploading this to inspire designs for the long-suffering significant others of Makerbot owners. Greg Frost made a valiant effort at making an openSCAD heart out of primitives for his broken heart http://www.thingiverse.com/thing:4683, but he wasn't satisfied with the small discontinuities, so I made this new and improved version. It's an implementation of Taubin's equation: http://mathworld.wolfram.com/HeartSurface.html.
While not printable itself, this heart is meant to be exploited for Valentine's mashup designs (in the spirit of http://www.thingiverse.com/thing:6078 and Makerblock's blog post). I hope to see this used in some entries to Pattywac's latest challenge: http://www.thingiverse.com/thing:6035.
The heart is 100x100x60mm, so scale from there. It has a lot of facets, so if rendering or skeining is going slow, it might be helpful to reduce the mesh in meshlab or similar. I've also included the script I used to generate it in Matlab (which might also work in Octave, if you prefer a FOSS alternative). You'll need the surf2stl script from the Matlab file exchange, but that's a great tool for modeling anyway.