Plug-ins are user-made expansions of the EVN universe, ranging from as small as simply making a Shuttle cost one more credit to changing the entire universe. Plug-ins use a custom-made file type as opposed to a common one. Specifically, on a Mac the file type is "EV Nova Plug-in" while on Windows it uses a .rez format. The latter is newer and more stable, as it doesn't require specific compression methods to avoid loss of data when transferring across the internet or converting. It should be noted that, while the Windows version of EVN comes with a Mac to Windows convertor, the Mac version does not have one for vice versa. However, there are tools available on the Mac, such as Mission Computer, that can convert both ways, which Windows cannot do.
There is a common distinction between the so-called "Total Conversions" (TCs) and the other plugs because a TC is of greater scale generally, namely in that everything is different from the original "novaverse", though in the view of some developers (Martin Turner and Peter "Pace" Craddock to name a couple), this distinction has no substance.
TC or Not TC?
Because of the TC/non-TC distinction, UE_R&D (Retribution lead developer) came up with the term "quasi-TC" as a way of indicating large plug-ins such as ARPIA2 and Aftermath. It is interesting to note the difference in size between Edward's Teacup TC and these two "normal" plug-ins, as this TC is so small it fits in far less than 100Kb.
However, there are many developers with little experience who become ambitious and announce new total conversion projects without much idea of how much work is involved. Many of these do not get very far and become vaporware. This is, unfortunately, a common phenomenon, causing many to take new TC announcements with a grain of salt.
There are several plug-in editors available, namely MissionComputer and ResEdit (with Novatools) for Mac, and EVNEW for Windows. Plug-ins are made of resources, each one representing a ship, outfit, mission, graphic, etc. If you want to develop your own plug-ins, the first step is reading the Nova Bible. It contains explanations of what each resource does and how to use it. The Utilities section of Ambrosia Software's add-on pages contains many useful programs for developing plug-ins. If you need help, EV-Nova.net and the Ambrosia EVN Webboard have forums dedicated to plug-in development.
It should be noted for developers used to working with more modern games and tools that EVN and its plug-in architecture are heavily based on the original Escape Velocity Classic engine, which was released back in 1996. Therefore, there are many limitations to plug-ins, most notably the lack of any sort of scripting, inability to add new features (such as A.I. types, weapon behaviors, etc.), limits on plug-in size (usually worked around by splitting a plug into multiple files), the requirement to use specific graphic and sound types (particularly antiquated System 7 format for sound effects), and seemingly arbitrary maximum and minimum resource values, all of which made perfect sense ten years ago. Even still, EVN's plug-ins can still be quite powerful, especially if one is willingly to manipulate their resources and the values within them to get around limitations or to achieve new, interesting effects.
Mac plug-in developers should keep in mind that, since the plug-in technology is old and relies on resource and data forks to store data, specific care should be taken so that data isn't lost. Mac plug-ins can become unstable and risk corruption when approaching 15 megabytes (which is extremely rare outside of lots of graphics for a large plug-in or TC), though this can be avoided entirely by splitting the plug-in into "parts", aka multiple files. Additionally, only certain methods of compression will preserve the data fork. The most popular and easiest to handle method is to .bin.zip the plug-in file via Plug-In Archiver. This method saves a lot of headache both for developer and the players when they try to decompress the file. Other working file types are .sit, .sitx, and .hqx, though these are less safe and can be very difficult or impossible for a Windows user to extract. .dmg is currently untested, perhaps due to the fact a Windows user could not use it. Windows developers do not have these issues and can simply .zip their resulting plug-in, as they .rez format is far more stable, though they should still keep plugs smaller than 15 megabytes so Mac users can use them without the files becoming corrupt.. A Mac developer could also take advantage of the .rez format by converting their plug-in to that file type before compressing.
Though with the advent of .bin.zip compression and Mac native Mac to Windows and vice versa conversion making it unnecessary these days, some developers still choose to upload separate Mac and Windows versions.