When comparing Terasology vs Freedoom, the Slant community recommends Freedoom for most people. In the question“What are the best open-source games?” Freedoom is ranked 38th while Terasology is ranked 79th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Well written in Java
Java is well documented.
Pro Nice looking
Shaders, bloom, film grain and other effects can be enabled.
Pro Free sandbox alternative
The game is inspired by other titles like Minecraft and Dwarf Fortress.
Pro Fully moddable
In the game you can create a mod and submit it as a module. You can select multiple modules when you make a new world.
Pro Extremely modular
The engine allows mix-and-match of features that you need.
Pro Active community
The community is very active in the development of the game, they participate regularly to Google Summer of Code and Google Code In.
Pro Can be Minecraft competitor
An open source alternative, fully customizable; easy mod support based on module pieces.
Pro The Doom feeling without the proprietary data files
Pro Thousands of available mods
Pro Easy mod creation
Making new levels and even gameplay mods is fairly easy and requires no programming knowledge.
Pro Easily moddable
You can apply mods others have made in a straightforward fashion. Find a mod you like (either from a quick google search or checking moddb for some) and the individual mod pages have a "tutorials" tab that you can use to figure out how to install it. It differs from mod to mod, but in most cases, it's fairly simple.
Pro Well documented modding possiblities
There are numerous tutorials providing detailed help with the modding and wikis providing extensive information on all aspects of the game