Rei's Mini map enables a view above the world in the corner of your screen (an improvement on the maps feature). You can also set waypoints and remember deathpoints. By pressing the (by default) "." or "~" key, you can view a list of options. Macs may have issues. You can easily edit the keys from the config folder or in-game in the menu. To do this on a Mac, go to Mac HD > Users > Username > Library > Application Support > Minecraft > mods > rei_minimap > keyconfig.txt. You can find the menu hotkey and set it to whatever key you want.
When this feature is enabled, a nameless waypoint will automatically be generated at the point where you were killed. These function as normal waypoints, and must be manually removed.
A very useful feature of Rei's mini map that is not well known is the slime chunk option. If selected in the options then all slime chunks are shown in red. Very useful if you want a slime farm. (This will not work for magma cubes as they can spawn anywhere in the nether but works for thaumic slime spawners as they only work in slime chunks)