There's hundreds of homebrew ROMs to try out. Here's a few favorites but don't let that stop you from seeking out your own fav's.

• https://www.zophar.net/pdroms/nes.html Has public domain ROMs
• http://www.nesworld.com/article.php?system=nes&data=neshomebrew has more NES homebrew games.
• Google for 'NES Homebrew'!

2048

This port of the popular & addictive sliding-tile game is a DIY ROM by tsone and it's fun to play & learn from.

Download from here

Assimilate

An alien adventure game, where you play the alien attacker!

Download from here

The Wit.NES

A homebrew port/tribute to The Witness - this game is a great puzzler with mysterious rules and an island with unknown history...

Download it here