Battle for Wesnoth is fun, and there are many great add-ons available. I particularly recommend the SX, RPG, and similar maps, which give you a single strong unit each (single-player or multiplayer co-op) that advances and levels up, facing a huge horde.

Dungeon Crawl Stone Soup is great if you enjoy roguelikes. It's designed to avoid a lot of the manual tracking needed in other roguelikes; for instance, you can search for where you left an item, or hit one key to explore unvisited areas of the current floor until you see anything unusual.

Hedgewars is a clone of Worms, if you're familiar with that.

Shattered Pixel Dungeon is fun, and available on mobile devices.