Lifthrasil: ...I will also include instances of 'soft' or 'only cosmetic' DRM, that don't really affect the gameplay...
AstralWanderer: In this case, I would highlight
Tales of Maj'Eyal. While the game itself is GPL-ed open source, an online connection (and account with te4.org) is required to gain access to extra features like
in-game special events and
cosmetic addons (which should be possible to provide for off-line players).
To clarify, the GOG version of Tales of Maj'Eyal
does include several "contributor" addons which the free download from te4.org does not (the Possessor class, Exploration mode, custom tiles, 3 vault spaces for transferring items between characters) - but not all of them.
Thanks for notifying me. I added it to the list.