Available Modifications
Listed below are modifications that advanced users may chose to impliment into the rendering of overviewer. These mods may include additional information overlays into the output, the style in which tiles are rendered, or additional actions that are performed by overviewer on each run. These typically require the administrator to have a little more knowledge of programming languages... as a note to developers of these mods, please feel free to update this list with links to your own scripts and mods.
Minecraft Sundial https://github.com/sunkid/minecraft-sundial/ - Will show your ingame server time as an overlay onto the Overviewer Map.
PlayerMarkers https://github.com/pironic/Minecraft-Overviewer-Addons/tree/master/PlayerMarkers - A bukkit plugin and overviewer javascript file to show ingame player positions live on the rendered Overviewer map.
Player Avatars https://github.com/ethzero/Minecraft-Overviewer-Addons/tree/master/Player-Avatar - PlayerMarkers actually uses this php script, if you would like to host your own copy instead of relying on the built in host that comes with PlayerMarkers, then use this script.
WorldGuard2OverviewerRegion https://github.com/pironic/Minecraft-Overviewer-Addons/tree/master/WG2OvR - This php script takes the regions defined in WorldGuard, and outputs them in isometric cubes on your overviewer map.
MCWeather https://github.com/pironic/Minecraft-Overviewer-Addons/tree/master/mcweather - Add a current weather and forecast icon to your Overviewer map.
Menus http://code.google.com/p/overviewer-menus/ - Allows other addon developers to create simple menus.
Re-Packaged Overviewer (with Add-ons) https://github.com/FabianN/Minecraft-Overviewer - This is a fork of overviewer with add-ons included. Comes with MapMarkers, Player Avatars, Sundial, WG2OvR, and mcweather. All you need to do is point WG2OvR, mcweather, and Sundial to the location of your regions.yml and level.dat files (details are in the README).