Plants vs. Zombies is an unofficial port of Plants vs. Zombies, made in 2012. It was likely developed by Nice Code.
Plants vs. Zombies utilizes the same engine as Angry Birds, and a lot of its music was recycled from said game. This port obviously has fewer plants and zombies than the original, but is considerably more faithful than the Angry Birds port. Plants vs. Zombies has five levels based on the first five of the official release.
Unused content
Plants vs. Zombies has some unused content, most of which are likely unused due to the rushed programming.
The game normally only has five levels. However, there is the unused Level 0 and Levels 6-9. They can be accessed by setting 0x600 to a value based on the level number (e.g. the value 07 sets the player to level 7). Aside from Level 9, the player is redirected to the title screen after beating one of the unused levels; completing Level 9 sends the player to Level 0. The numbers 0, 6-9 even have an unused sprite, given the levels aren't accessible.
A pause function was never implemented in Plants vs. Zombies. The similar game Angry Birds did have a pause option, and therefore the pause sprite went used. The sprite is misspelled "PAUS".
There are unused sprites for a decaying Walnut, a squished Squash, and even an unused animation frame for the sun. There is even an unused sprite of a Puff-shroom, which appears in later stages of the official Plants vs. Zombies.