The vast majority of kydex knife sheaths on the market are fold over designs or simply two pieces of kydex sandwhiched over the knife. Both styles are typically secured with large rivets that, while handy for attaching things, add a lot of width to the sheath which can make them less comfortable for certain types of carry.
Blade Rigs can make you a sheath like the ones mentioned above but they also build some really slick purpose built sheaths that are very unique. Their IWB and Pocket sheaths both use smaller rivets that really cut down on the bulk of the sheath. This makes them much better suited to their intended carry methods than typical sheaths.
The Blade Rigs IWB Kydex Sheath features a steel clip that can be used to secure the sheath in a number of ways. They can be attached to belts of various sizes, MOLLE webbing and even belt loops. The sheath is molded so that the knife can be inserted with the edge oriented in either direction. These sheaths seem to be very well suited to center line carry for some classic back up knives like the Gerber Guardian, Cold Steel Braveheart, and the Gryphon M-10.
The Pocket Sheath features a hooked cutout that is used during the presentation to snag the sheath to remove it on the way out of the pocket. The sheath is stippled to serve as a push off point for removing the sheath and as an index point so you can tell by touch how your knife is oriented.
You can find more information at BladeRigs.com.