Because these games are available online, the charm of these games is that as long as you have the means (e.g. a personal computer and an Internet connection) you can obtain them whenever you like. Many of these games exist, from those that are so detailed it's hard to believe that you can play them for free to those that never got out of their testing stage. Sturgeon's Law applies. Some of the best of these have received commercial releases.
Also, because they are free, many of these games have extremely unorthodox components, and have a completely different air to them than commercial releases. If you search through titles you haven't heard of, you may find games created for very niche audiences.
Just like commercial games, many of these games have bountiful fanworks uploaded to pixiv. In recent years, more and more games are garnering attention through Let's Play videos uploaded to Nico Nico Douga and the like.
The definition of free games also encompasses browser games and Flash games. See also Doujin Game.
A List of Freeware Games
※Bolded titles have over 100 uploads; titles with a ★ have over 1000