Rei's Minimap is very useful in any Minecraft world. It is currently in the Voltz modpack. The map lets you set waypoints wherever you want, so you don't get lost and can always find something again. It adds many useful things, such as: coordinates, entities radar, a view of your land or creation, and a big map. It can also show your death point to find your corpse/left-overs again.
To use you can press the . key (period key) to expand the map. In singleplayer you press the M key. Though this can be changed in the "Keyconfig" menu.
The colors of waypoints can be customized as well as being able to change their names. You can turn waypoints ON and OFF.