In a manner reminiscent of the feature from ChickenBones' NEI, Chunk Edge Indicator shows the player where the chunk boundaries are.
Pressing the F9 key (configurable in the controls menu, under Miscellaneous) will toggle between three states: Off, Corners only, and All.
This mod is Client Side only (servers will ignore it if provided).
As of Minecraft 1.10 this mod is pretty much obsolete due to the F3+G vanilla functionality. I updated anyway because it was pretty trivial to do.