Old versions
The current version (1.2.5 released on February 16, 2018) is provided in three different flavors: desktop (the main and the most powerful one and available for GNU/Linux, MacOSX and Windows), web (that you can run online on Chrome/Chromium thanks to our plugin) and Chromebook, which has some limitations but is still supported because of the amount of Chromebook users in the USA.
You can always look for previous versions on GitHub releases.
The embedded version is also available on GitHub, but hasn't been updated since Snap4Arduino 1.2.4. This shouldn't pose any problems as most modifications after 1.2.4 have no implications in a GUI-less version. The embedded version works in any GNU/Linux based machine, including embedded platforms and single board computers. You can also run it on new generation Arduino boards (Yun, Tian, Industrial 101...) and also on any kind of Gnu/Linux based single board computers (Raspberry Pi, BeagleBone...). Note that the most powerful single board computers can also run the regular web version. More documentation on this section of this wiki.
An experimental 1.2.3 version for Android was released in 2017 but never updated afterwards because of the huge amount of work that keeping it up to date required and several UI issues that couldn't be fixed. You can still find the old version on GitHub, but note that you will need to communicate with your board via Bluetooth. Check out this section to read about Bluetooth communication.
Windows XP has been discontinued even by Microsoft. The latest version of Snap4Arduino that could run in it is 1.1.6-beta-RC2, but you can always run the web version using Chrome, while Chrome is still supported in XP.