* This feature is available only when programmed for a specific game by its developer.
† Customized graphics are normally available for any game, but when a game uses a specific board image, or there is only one piece set matching the pieces used in the game, customization options may be more limited or unavailable.
The main advantage Game Courier has over other play-by-mail systems is its versatility and customizability. Other PBM systems will lock you into a closed set of games, but Game Courier will let you play nearly any Chess variant. Even when no one has made a game available for Game Courier, you can still set it up yourself and begin playing it right away. One thing I wanted with Game Courier was an easy way to play my own games online. The features that let me play my games online are also available for you. You can design boards, program game rules with the GAME Code language, and start playing your games with anyone who is willing. You're also free to customize the graphics, to make modified clones of games programmed by other people, and to just setup a board and play games without programming them.
Because of its versality and customizability, Game Courier currently supports over 700 games. Most of these have been provided by users, not by me. There are also many top quality games here, including many of my own that you won't find on any other PBM system, such as Eurasian Chess, Extra Move Chess, and Kamikaze Mortal Shogi.
Game Courier's main drawback, compared with other PBM systems, is that it does not enforce the rules for all the games it supports. But it can and does enforce the rules for many games, and its ability to support games without rule enforcement allows it to support many more games than it otherwise would.
Zillions of Games is an excellent program for playing Chess variants and other strategy board games. Like Game Courier, it is user-expandable, letting users design and program many different games. The main advantage of Zillions of Games over Game Courier is that it will provide you with a computer opponent. On this strength alone, Zillions of Games is worth getting if you have a Windows OS or can run Windows program with Wine or another Windows API. No other program for playing Chess variants against your computer is as versatile or as customizable as Zillions of Games. Game Courier is as versatile and as customizable as Zillions of Games, but it won't provide you with a computer opponent. Compared with Zillions of Games, its strengths lie mainly in how it is better for playing games against human opponents. It more reliably connects you with human opponents, allows you to play games with more people, and keeps track of all your games for you. Also, its programming language is more of a full-fledged programming language. It includes features lacking in the ZRF language and allows support for games that can't be played with Zillions of Games. For developers who aren't artists or programmers, Game Courier has the advantage of allowing dumb presets that don't enforce rules and of drawing your board for you. This makes development easier than it is for Zillions of Games.
When you play a game by emailing ZSG files, your game will be disrupted if an email isn't delivered. But with Game Courier, you can check on your games even if you don't get any email. When you use Zillions to play a game in real time, your game will be disrupted if your computer crashes or your opponent's does. But with Game Courier, you can play in real-time without worrying about this, because crashing your computer won't affect the log of your game. You can alternate between playing a game in real-time and playing it asynchronously. If your session is disprupted by a computer crash or another event, you can just continue it later. When you move in Zillions of Games while playing in real-time, it is possible to make the wrong move because of a slip of the mouse. With Game Courier, you can more reliably enter the move you intended, because it requires moves to preview and confirm your move before logging it.
Zillions of Games is a commercial Windows program. This limits your opponents to other people with the Zillions of Games program and a Windows OS. As a server-side web-based program, Game Courier will work for you no matter what operating system or web browser you use. This expands your range of potential opponents to Macintosh users, Linux users, Amiga users, etc.
The ZRF language used by Zillions of Games is very limited in its capabilities. It has no math, no string manipulation, no functions, no subroutines, and limited control structures. In contrast, GAME Code is a Turing-complete programming language with math, boolean, and string operators, user-defined functions and subroutines, and all the usual control structures normally found in other languages, as well as various functions and commands designed specifically to aid rule-enforcement in Chess variants. It generally takes more programming skill to program game rules in GAME Code, but more can be done with it. It can be used to program rules for games that can't be programmed for Zillions of Games, such as Marseillais Chess. It can easily be used for games that could be done in Zillions of Games only with large, bloated code, such as Xorix Shogi. It can handle the finer details of rules that Zillions of Games can't, such as the Shogi rule against checkmating the King with a Pawn drop. It can easily be used for games with random elements, such as Vegas Fun Chess, and it has a set of commands for using cards, which can allow games similar to Knightmare Chess. This is not to say that it is more versatile in every respect. Game Courier cannot support multiplayer games, whereas Zillions of Games can. But other than that, Game Courier is more versatile than Zillions of Games.