As we've already mentioned, the specifications of the kit lens and 50mm's for all three cameras are virtually identical. All three 18-55mm have F3.5-5.6 aperture ranges and no distance scale, and all three 50mm's are F1.8. None of the lenses come with hoods. All six lenses have plastic lens mounts, and none of the six support full-time manual focus (what Pentax calls "quick shift"). It's almost as if Canon, Nikon, and Pentax have agreed that the consumer-grade kit and prime lenses will all share (or lack) the same features.
The Canon EF 50mm F1.8 II retails for under $100 and is the cheapest of the three 50mm lenses. It focuses very quickly but feels light and plasticky. There's nothing wrong with the image quality, though we did not perform in-depth tests (plenty of other sites have done this already). The Canon 50mm is slightly larger than the other two lenses.
The Canon EF-S 18-55mm F1.3-5.6 IS II kit lens also feels light and cheap, and the zoom ring feels very loose. Overall, the Canon lenses are quite noisy when they autofocus, but the AF is still very fast.
The Nikon 50mm F1.8D retails for under $125 and is still a bargain. However, it does not support autofocus on the intro-level Nikon bodies (i.e. those that only support S autofocus), including the D5100; one would have to upgrade to a 50mm F1.8G, which costs approximately $220. The D version of the 50mm is nevertheless very compact and well-built.
The Nikon AF-S DX 18-55mm F3.5-5.6G VR has the best build quality of the three zoom lenses. The zoom ring is very smooth and there is no play whatsoever. The autofocus is practically inaudible and on par with that of the Canon lens in speed.
The Pentax-DA 50mm F1.8 a recent addition to the Pentax lineup, and was practically launched alongside the Pentax K-30 this summer. It retails for approximately $250, making it the most expensive of the three 50mm's. It features screwdrive autofocus with very short throw, meaning that it is capable of focusing faster than Pentax's other 50 and 55mm prime options. It is the smallest of the three 50mm lenses.
The zoom ring on the smc Pentax-DA L 18-55mm F3.5-5.6, similar to that of the Nikon lens, feels rather firm, and there is little play. The autofocus is a little bit more noisy than that of the Canon lens, and both lenses felt a bit slower.
We did not test the image quality of the three lenses, but given their similarities, we expect there to be no significant difference in image quality.
The Nikon lenses (if we look at the 50mm F1.8 G rather than the D) are the most balanced overall, with silent autofocus and the best overall build quality. The Nikon 50mm is priced in the middle. The Canon lenses feel cheap and cost the least overall, but despite this the 50mm focuses the fastest. The Pentax lenses are in the middle in terms of build quality, but they are the most compact. Their AF is comparatively noisy and they are the most expensive. Unlike the other 50mm's, the Pentax lens enjoys up to 4 stops of stabilization when mounted on the K-30, however.
All three zooms are crop-format (DX / APS-C) lenses, while the Canon and Nikon 50 mm lenses are marketed as full-frame (FX) compatible.
The Canon and Nikon zooms have built-in stabilization, while all Pentax DSLRs come with stabilization, meaning that the lenses tie in this respect (and this together with the screwdrive AF explains why the Pentax lenses can be made smaller).