We steal Star Wars Saga Edition's Force Points in place of Inspiration. You get 5+1/2 level, rounded down, per level. You can use it to add 1d6 to any one d20 roll.
I'm considering the Condition Track as something to combine with the Exhaustion system. Basically it's just a chart for Exhaustion, converted into 5e mechanics, so this isn't so much a houserule.
All characters get a "bonus feat" at 1st, 11th, and 17th character level. Cannot be a directly combat related feat.
All characters gain +1 to 1 stat of their choice every time they get an ASI or feat. at my table, this makes people feel less like they have to take ASIs. Hasn't actually had much impact on balance, so far. I'm ready to adjust to any problems that arise, but so far it's fine.
All characters that normally only get 2 skill proficiencies from class instead get 3. Class skill list remains unchanged.
Add the Riddle skill, which is based on and similar to the oen from Cubicle7's Adventurer's Guide to Middle Earth. Speaking in code, deciphering linguistic codes or riddles, and figuring out what the heck some esoteric codex is talking about are all Riddle challenges. Usually an Int skill, but occasionally Wis or Cha apply.
I use Skill Challenges per the 4e Rules COmpendium.
Not so much a houserule: Most magic items that aren't made by the PCs or their allies are used against the PCs by their enemies, first. Humanoid enemies stay dangerous a lot longer when they can have wands of fireball, and wands of fireball are more exciting when you pick them up from the dead hands of an enemy rogue who used them against you.
Also, in the Xanathar's Guide rules for crafting magic items, a rogue3/wizard1 like my Gnome swashbuckler/tinker/mage slayer could make a magic item that casts 3rd level spells as long as his team is good and creative, and the GM isn't trying to stop it, because they just have to overcome a CR 1-4 (IIRC) critter to get the rare materials.
As much as I think that "deal with a CR x monster to get the rare materials magic items always need" thing is trite and gamist in the most lame way possible, it also forces the DM to come up with quests just to make an item, and creates the weird possibility of a DM having to either retcon something, fudge something, or allow a magic item that is way over level into the campaign. For DMs that view that level of DM control over what can even be tried as bad DMing, that doesn't work. IT also doesn't work in any world where magic items aren't super rare.
So, we don't require getting around a monster/NPC of a given CR. We do use the CR chart as a gauge of how hard it should be to find/negotiate for/whatever the materials, and we keep the costs pretty much in place, but we also want to use character level as a limiting factor.
There are two ideas we are discussing.
1. If the spell is higher than your caster level would be as a full caster, you have to make a crafting check, using Arcana or the tool proficiency, as part of crafting the item. DC is 10+spell level, and the check must be made once per workweek. If you fail a check, you lose that week's work, including an amount of gold equal to cost/number of workweeks. If you fail 2 checks, or half the checks needed (whichever is higher) to craft the item, you must start from scratch, and you waste the rare ingredient.
2. If the Spell is higher [same as above] you must make 1 crafting check with either Arcana or the tools used in crafting, DC as above. If you fail, you waste the rare ingredient, but can salvage the other materials and components, and waste half the time it would have taken to craft the item.
2a. If you roll a 1, you also waste half the mundane materials and magical components (ie, the gold).
If you can't tell, we also don't like that crafting requires no checks. We're also open to DC 15+spell level, but we want people who don't have expertise and maxed stat to have some chance to succeed.
In either case, you don't have to make the checks if it's a spell level you would have access to as a full caster of your level.