One of the most popular and most desired games for this system, I learned of its existence only recently.  Tunnels of Doom is an old game, and it shows, but still a classic for the people who once owned this computer and the only RPG available for the TI (that I know of).  It was supposed to support expansion disks with more quests on them, but to the best of my knowledge these never materialized, leaving only the two original quests, Pennies and Kings. See also the Adventure International page for more TI games.