Best free "ASCII games" games

A few decades back, ASCII games (or text-based games) were quite popular. Why? Because graphics on home computers were not that good. Fast forward a few years: the appeal of ASCII games didn't fade completely. Ignoring almost completely the visual aspect of a game allow the developer to focus on gameplay, depth and complexity. Dwarf Fortress and ASCII Sectors are two examples of fairly complex text-based games.
And then there are MUDs: Multi User Dungeons. MUDs are text-based really old-school multiplayer RPGs with even less graphics than ASCII roguelikes.
ASCII gaming is not for everyone, but you may be surprised by some of these games; give them a try.