If you want to play ITG, you have several options as far as the software is concerned.
The original In The Groove arcade game ran on a proprietary fork of StepMania 3.95. Since then, newer versions and forks of StepMania have released. While older versions are still an option, it's generally recommended to switch to a version that is being actively maintained.
For specifically dance pad ITG players, we recommend:
This video gives an overview of some of the popular forks, but it does not cover ITGmania.
Project OutFox is a fork of StepMania 5 (SM5) that was originally positioned as the eventual new main branch for StepMania. When compared to its predecessor, OutFox is extremely performant, especially on lower-end systems. Many new features and game modes are officially supported in OutFox, and a dedicated online ranking system is currently in the works. Additionally, the project also includes freely-distributable songs and stepcharts, known as Project OutFox Serenity.
Since its inception, it has been closed-source, meaning only those directly involved with the project have access to the source code. It is supposedly planned to become open-source in the future when it is merged into the StepMania main branch. How Project OutFox intends to merge with the old StepMania source isn't clear, though it will most likely sport a different, less-permissive license.
StepMania 5.1 Beta 2 (5.1b2 or SM5) is the latest stable version of official StepMania. On the official StepMania website, an older version (5.0.12) is listed on the downloads page, but the newer 5.1b2 can be found on their GitHub.
SM5 development has in large part halted. SM5 is open-source and has a very permissive MIT license.
Some users recompile and distribute their own builds of SM5 that have a handful of small fixes. For example, this is a portable build of SM5.1b2 built by Din that has the "mine fix" applied.
For arcade cabinet users, you may have heard of TJ's system image, which uses SM5.1b2.
StepMania 3.95 was the popular long-lived branch that preceded SM5 and is what ITG (AC), OpenITG, and NotITG are based on.
Most builds have been archived, and can be downloaded here.
ITGmania is a newer fork of SM5 that is focused more narrowly on 4-panel pad players. It is open-source and provides builds that work on all major operating systems and comes pre-bundled with the most popular ITG theme Simply Love. It also has some other nice features like built-in networking (for GrooveStats score tracking) and better integration with Stamina RPG.
In The Groove (ITG) was an arcade dance game developed by Roxor Games, and built off of a closed-source fork of StepMania 3.95. Compared to its contemporary Dance Dance Revolution, it introduced new features like mines, unique modifiers, USB profile support, and a distinct charting style.
ITG originally released as an upgrade kit for DDR cabinets, then, as its own dedicated arcade machine. Later, it was released for the home market on PlayStation 2, and PC.
Eventually, Konami filed a lawsuit against Roxor Games, claiming unfair competition, and patent infringement. The lawsuit ended in a settlement which resulted in Konami acquiring the intellectual property rights of In The Groove.
OpenITG is an open-source fork of StepMania 3.95 that tries to recreate the ITG-like features from official In The Groove. It also served as a drop-in replacement for the ITG binary on arcade cabinets.
NotITG is a closed-source fork of OpenITG that runs only on Windows. It is primarily focused on mod charts with crazy visual effects.
Club Fantastic is an all-in-one download that includes the Club Fantastic simfile packs as base content, with two major goals: non-commercial music that can be freely downloaded and streamed, and ease of install for new players.
As of the Club Fantastic Season 2 release, the StepMania build included in the package is based on 5.1-b2 (mine fix), with some additional tweaks (such as storing player profiles next to the game itself, instead of in %APPDATA% user folders), and the included theme is based on Simply Love. Future development will be more aligned with ITGMania.