Grid showing chunk borders
/grid lines /grid chunks will give you orange lines where chunk borders are.
Grid showing a block pattern every 8th block to show where torches should be placed
/grid blocks /grid 8 /grid here When you want to space out torches, or anything else, every X-th block, grid can highlight those blocks.
Grid showing circles to help building round structures, seen from above
/grid 20 /grid circles /grid fixy (before flying up) In circles mode, grid shows which blocks are closest to a circle with a given radius.In this example, grid height is fixed to the ground, not floating with the player.
Grid showing possible mob spawn blocks
/grid spawns This highlights blocks that are dark enough to allow mobs to spawn - red highlights mean mobs can spawn anytime, yellow is for blocks that are dark enough only at night.
Torches every 5 blocks light up an area enough to prevent mob spawns
/grid 5 /grid blocks /grid spawns Grid helps you to place torches in a regular 5x5 pattern to light up everything enough so mobs can't spawn.
Building a road
/grid blocks /grid 3 7 Different X and Z coordinates; grid shows where to place lampposts when building a road.
Hex overlay
This is the new (since 1.6) Hex grid overlay. /grid hex, /grid lines.
Example how hexes coords work
With grid hex, the distance numbers are the blocks until the pattern repeats - this is not the "diameter" of the hex!. To create this, you'd need to use /grid 15 9, with the 9 being rounded up from 8.7.