GOG is giving away Mount & Blade for a limited period of time. It is available for about 40 more hours, so grab it as soon as possible.
To get it, go here and login to your GOG account. Then click the "GET FOR FREE" button and it's done (really, it's that simple). The game will be added to your library and will be accessible forever.
P.S. GOG only deals in DRM-free games. So any clients or something isn't necessary to play its games.