Arx Libertatis is a cross-platform, open source port of Arx Fatalis, a 2002 first-person role-playing game developed by Arkane Studios. Arx Fatalis features crafting, melee and ranged combat, and a unique casting system where the player draws runes in real time to effect the desired spell. The Arx Libertatis source code is based on the publicly released Arx Fatalis sources. This does however not include the game data, so you need to obtain a copy of the original Arx Fatalis or its demo.
Pioneer is a space adventure game set in the Milky Way galaxy at the turn of the 31st century. The game is open-ended, and you are free to explore the millions of star systems in the game. You can land on planets, slingshot past gas giants, and burn yourself to a crisp flying between binary star systems. Pioneer has a sneaking resemblance to Frontier: Elite 2.
Naev is a 2D space trading and combat game, in a similar vein as Escape Velocity. Naev is played from a top-down perspective, featuring fast-paced combat, many ships, a large variety of equipment, and a large galaxy to explore. The game is highly open-ended, letting you proceed at your own pace.