VCS is a visual d20 3.5 combat program for dungeon masters. If you have dual monitors, one can show the DM map and the other the player's. It is possible to 'hide' things, and the players will only see what's in range or what they've seen before. After drawing the map, players can be moved, and over 850 standard monsters selected from for them to fight. Huge maps can be drawn and filled with monsters very quickly.
Proelia is a multi-game system encounter tracker for pen-and-paper RPGs. It allows you to create "prepared" encounters for later use, and then run them any number of times with the click of a button. A group can be created from an encounter's participants, and then re-used in other encounters. Each encounter can display a map that allows you to track positioning of participants, especially in relation to regions, so you can tell if someone is standing too close to a fire, or is subject to an aura or other environmental effect. A color and marker text can be set for each participant in the encounter, so you can quickly find them on the map. A variety of RPGs are supported through a plugin system that allows new systems to be added.
Arx Libertatis is a cross-platform, open source port of Arx Fatalis, a 2002 first-person role-playing game developed by Arkane Studios. Arx Fatalis features crafting, melee and ranged combat, and a unique casting system where the player draws runes in real time to effect the desired spell. The Arx Libertatis source code is based on the publicly released Arx Fatalis sources. This does however not include the game data, so you need to obtain a copy of the original Arx Fatalis or its demo.
EZDM (ezdungeonmaster) is a simple set of tools for dungeon managers, targeting AD&D 2nd edition rules. It's not an RPG game, but some helper tools for people DM-ing real games, and is designed not to get in the way of roleplay and storytelling, but simply help do the math during encounters and leveling. Dice rolling can be done by the computer, or the tools can be set for manual dice, in which case you simply enter the results of the roll. All game data, charts, and character sheets are stored in JSON format. The EZDM kit defaults to using a simple dialog-based GUI which requires easygui, but a console interface is also available.