As a diff or merge tool
This works for most of JetBrains' IDEs, including IntelliJ IDEA (even the free community edition), PyCharm (even the free community edition), and CLion. It might make sense to install one of the free IDEs just for the merge tool.
Here is a snippet for your config, using IDEA as the example:
[merge-tools.idea]
# Likely needs to be adjusted, see below
program = "idea.sh" # Linux
program = "/Applications/IntelliJ IDEA CE.app/Contents/MacOS/idea" # MacOS
diff-args = ["diff", "$left", "$right"]
edit-args = ["diff", "$left", "$right"]
merge-args = ["merge" , "$left", "$right", "$base", "$output"]
The "program" field will likely need to be adjusted. See command-line interface docs for IDEA for information. The other IntelliJ IDEs have similar instructions; only the name and the path to the binary change.
https://plugins.jetbrains.com/ has some nice plugins. For example, there is a plugin to compare images.