(I have no idea what this is, but it has been requested by someone in the SourceMod Discord channel.)
Commands:
!marker - opens the marker menu.
+marker - equivalent for "draw marker".
Markers can be only placed by team 3 (CT in CS:S/CS:GO, Blue in TF2) players or admins.
The color can be customized and the default setup contains: white, pink, orange, yellow, lime and light blue.
When the marker is hidden, it is only visible to the player who marked it.