Thanks to its openess, OpenArena has been "ported" to several Operating Systems. And even for the three classic PC OSs Windows, Linux and OS X (for which official downloads are available from OA official site), it is also possible to get the game though some third-party "markets" (a.k.a. "app stores" or "digital distribution platforms"), like Desura. OpenArena Team does not have control over those third-party developers. While OpenArena itself is fully free, a small part of such third-party publishers may ask for money (GPL license allows commercial use, so they can do it).