Publisher: AtariAge

AtariAge is a website for Atari players and collectors, with forums and a large database of games, manuals and rarity ratings. It also runs an online store that sells new cartridges, so almost everything under this name is a modern homebrew, written decades after the console’s heyday and usually sold with a manual and printed label art. Some of these games were released by their own authors or small labels and only sold through the store, such as Warring Worms from Billy Eno’s Baroque Gaming.
The site has 18 pages here, several of them NTSC and PAL versions of the same game. A few to try:

Halo 2600 (2010): Ed Fries’s take on Halo as a 1982-style game, fitted into 4 kilobytes.
Medieval Mayhem (2006): Darrell Spice Jr.’s four-player castle game based on Warlords.
Boulder Dash: Andrew Davie and Thomas Jentzsch’s conversion of the digging puzzle, made with First Star’s approval.
Lady Bug (2006): John W. Champeau’s version of the arcade maze game, with gates you swing to reshape the maze.
K.O. Cruiser (2010): Devin D. Cook’s first-person boxing game against six cartoon opponents.
Duck Attack!: Will Nicholes’s tribute to Adventure, full of fire-breathing mutant ducks.

No cartridge needed: pick one below and play it in your browser.