Rei's Minimap 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.
The waypoints feature will remember a position on the map by inputting a name for the waypoint, and choosing a color. The waypoint will be seen in the physical world and in the minimap. Coordinates for waypoints can be manually set as the default waypoint coords. You can delete them in your Minimap-Menu under "Waypoints" > "Delete".BHfhhndjsikdjfneisdfjwikscdkewls
When turned on, whenever you are killed, a waypoint will be placed on the map at the point where you died. Useful for finding your stuff if you died in a cave system. Technically they are just waypoints, which means that you can delete them like waypoints (see above).
A very usefull feature of Rei's minimap 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)