UPDATED V3 added - same as V2 except I have included a pair of towers and a bridge, this should test retraction settings as strings will form between the towers if your retraction is insufficient, bridge is a test of part cooling fan's ability to cool the filament enough to keep it horizontal.
UPDATED V2 added. So a few tweaks, I've made the 50mm x50mm part a little taller as use of brims, and the spread of filament on the first layer make measuring without a little extra height more difficult and less accurate, so measure from just above the first layer for x and y. I've included straight and rotated (for corexy) versions.
Okay so we've all come across Triffid Hunter's Guide (http://reprap.org/wiki/Triffid_Hunter's_Calibration_Guide) and in it he suggests using 5mm calibration cube steps (http://www.thingiverse.com/thing:24238) which is a very useful model. However I wanted something a little more accurate and able to do esteps fine tuning more easily.
So this object can be used for x and y calibration. Its 50mm x 50mm so larger and more accurate for adjusting x and y steps/mm than the typical 20mmx20mm cube. Z is only 10mm in the standard version to make it quicker to print, but I have included a version with a tower to 25mm for z calibration also, slightly too tall or too short might also help indicate under/over extrusion.
The next part is e-steps. Triffid Hunter's guide recommends printing an object with 95% infill and adjusting esteps or flow on the fly to the point where little gaps appear between infill lines. This however only sets one end of the fine tuning range for esteps. At the other end is the point at which infill becomes stringy.
Set infill in your slicer to 15% rectilinear so it can easily be seen. Then using Slic3r (other slicers might do this differently) "split" the model. The smaller cube can then be assigned custom parameters (right click and pick settings for the smaller cube) assign infill_density 95%.
Now print it, one cube will print lightly infilled so you can easily see if the infill is stringy and increase flow/esteps the other will print dense, but not complete, infill so you can reduce esteps/flow if you cannot see tiny gaps between infill lines. In this way you arrive at the mid point between slightly-under and slightly-over extruding. As per Triffid Hunter's guide don't start adjusting until you get to layer 5 or 6 as the amount of squash on your first layer affects the layer height too much until that point. A good point to begin to adjust the flow/esteps is once the outer 50x50mm wall has finished and the printer is just printing the two cubes, this is around z=3mm.
I have also included a set of thin walls. They are 0.3mm, 0.4mm, 0.5mm and 0.6mm. Slic3r prints these all at 0.4mm with my 0.4mm nozzle but gaps in these walls indicates slight under-extrusion, unless you hear some pops as they print then this is either gas escaping from the nozzle most likely water vapour, dry your filament out if this happens, or the temperature is too high and the filament is breaking down producing gas, lower the nozzle temp.
The A and B markings are for corexy printers, the model is rotated 45 deg so belt tension can be differentially adjusted (this can be avoided if you are using a balanced belt tension carriage like this one http://www.thingiverse.com/thing:1582049 and your two corexy belts are exactly equal in length). I have written a guide to easily adjusting x versus y here on reprap (http://forums.reprap.org/read.php?344,705441,705441#msg-705441) or use this guide if it makes more sense to you http://forums.reprap.org/read.php?397,530210 for adjustment of the belts.
The version with the z tower only requires around 6.5g of plastic (ABS) and takes around 50 minutes to print at 50mm/s avg print speed.