RJ11--common telephone wire--is common. Probably everyone on this board has a mile or so of the stuff laying around from various sources, and the connection is somewhat simple--line up the colors, and slip on the crimp connector.

Would there be anything to consider if using it for general plugging stuff together on a project? I'm not sure how much current it could take, but for situations where you want to be able to plug stuff in and out frequently, or swap boards and components easily, it would be a lot better than jumper wires or digging through Digikey for the pins and stuff to make your own servo style connectors, and would look neater too.

The big thing I was seeing was how easy it is to find the cable, and cut it down and put connectors on it.

The board-side connectors tend to be rather large.
There are at least two standard ways of making a cable, one of which reverses the connections.
Six wires is too many for some things, and too few for a lot of other things.
Actual telephone equipment that uses these cables can put inconveniently large voltages across wires, easily leading to damage if you put the wrong plug into the wrong jack.
For good reliability, you have to make sure that your connectors match your wire type WRT stranded/solid.
Other than that, it is pretty widely used. For example, RJ11 is one of the standard connectors used for ICSP in the Microchip PIC world...