(For convenience, I'll mark free ones with (F) )
Easy(ish)
>Dungeons of Dredmor
Build a class out of several skills (Weapon skills, some casting abilities, crafting, etc). These skills fall under Warrior, Rogue, or Wizard, and each level you gain in one of these Archetypes gives you stats along their specialties (In addition to what the skill gives you.) Lots of mods, though some add questionable, unbalanced content. Doesn't take itself seriously, but pokes fun in all directions. Not much internet may-may humor here, at least relatively speaking.
>Dwarf Fortress (F)
Listed as Easy because there's no real winstate and you set your own goals, the Adventurer mode basically acts as a roguelike. Fucking bananas combat system where each organ and bodypart has several layers (Skin, fat, muscle, bone) and HP. You build, basically, a setting from scratch each time you play, but you can also keep playing in the same world as much as you like.
Fortress Mode can be fairly easy as well if you know how and understand how Sieges work.
>Caves of Qud
MUTATIONS: The Roguelike, you're basically playing in the post-post-post apocalypse. Playing as a Esper (Mental mutations only) can be fairly easy, so long as you prioritize survival over immediate victory. Pure human takes a bit more work, but also yields impressive results if you go tech.
Middling
>DoomRL (F)
Doom as a roguelike. Turns are based on speed ticking, not hard turns, so faster things 'go' more often and slower things less. Things that have moved recently have an evasion bonus. Reloading takes time. Your character is basically built on feats you get each level, and you can customize your equipment to a degree. Very fun, and even more fun once you understand the flow and take your turns faster.
>Cogmind
Most Roguelike games function on a 'food clock'. (Qud, DF, and some others on this list do - DoomRL does not spawn new enemies over time and thus doesn't need one.) The game design function of the food clock is to keep you descending: Your character must eat, food is limited by floor, and so staying on one floor and farming for levels and items is not viable. Thus, you must progress, else face starvation, and face stronger foes. Cogmind subverts this by having the levels slowly scout you out, notice you, and send progressively stronger enemies your way. Work quickly, salvage fast, or die.
You play as a robotic "core" that can have various items strapped to it - these function as both armor and whatever else they do. Movement items determine your speed, weapons allow you to deal damage without ramming, and armor protects your core. There is also 'function' items that do shit like give you radar. You can also hack terminals for information, items, and later on, subvert the enemy 'economy'.
>Brogue (F)
Basically a shinier, prettier Rogue.
No, really. There's not much more to say about it. It's simple, to the point, and enjoyable.
>Dungeon Crawl Stone Soup (F)
Despite some questionable patches, this remains a very solid roguelike with some great mechanics.
>Elona+ (F)
Very freeform Roguelike with very little direction. Read some documentation on it, it's kinda impossible to explain here with any sense of brevity.
Hard
>Incursion (F)
A roguelike that uses 2nd edition DnD rules. Yep. Crashes often, but has some really interesting mechanics behind it. Casters are fucking bananas to play, and I'm not sure if the game actually has a wincon yet because it's not really done.
>Neo Scavenger
Not really well-designed and runs on Flash (Eugh). Only go for it for the novelty, and make sure to pirate it. Only hard in the sense that early death is hilariously common and total shit (YOU DIED FROM NOT HAVING SHOES, TRY AGAIN AND HOPE FOR SHOES? Y/N/FUCKTHIS )
>'Bands, 'Hacks, etc (Should be F, some ain't.)
Derivatives of Angband and Nethack. Most should be free, but I've seen some on the store. These games like to swing a big horse cock in your face branded MECHANICS and DEPTH and not stop until you've made sense of the way shit works.
Unique
>Infra Arcana (F)
Descend into a catacomb and try not to go fucking bonkers as you deal with the occult. Very fun, but hard as balls. Has a neat feature where you have to walk through a graveyard of all your past losses to get to the dungeon, not to mention sanity effects.