With my house 3.5e campaign venturing up onto the mythic plateau of the giants, an interstitial realm in the roof of the world, I thought it fitting to throw in some of the interesting house-rules I have found around the web to try and emphasise the 'land of myth and legend' atmosphere.
The first few are timely additions from Thalian Musings post on Houserules During Play
Big d30 – once per session per player, that player can choose to roll a d30 instead of the regular die for an attack, damage, saving throw, skill check, spell ability, or anything else a player can make a sound argument for. This is just fun, gets used a lot less than I expected, but has led to a natural 30 being rolled to great rejoicing.
Fleeting Luck - a riff off the Dungeon Crawl Classics idea - a Nat 20 rolled gets a luck token, which acts as a reroll of whatever you like. Anytime anyone rolls a Nat 1, all luck tokens around the table go away. You cannot use your luck to reroll that Nat 1. Every session all players start with one point, fleeting luck does not carry over.
Next I have modified More Risky Spellburn from Sheep & Sorcery to become Alternate Spellburn - a caster can choose to max out a spell effect; damage, HD turned, etc. by taking d6 spellburn to a stat of your choice. Can also be used to cast a spell you know how to cast but cannot normally do so again before rest. This is specifically to allow the sorcerer who is normally using the Spelljamming Saddle to fly the party about to play a more active role in combat if they wish.
On the DMs side of the screen I use the Overloaded Encounter Die - switched out from 4-in-20 for a chance of a random encounter to overloaded d6 for exactly the reason designers stated, got fed up with 'roll for nothing' - all rolls should be meaningful.
I also use Reaction Rolls and Morale Checks for monsters but these are hardly house rules, just gaps being backfilled from older editions. I like them as it makes encounters more than 'scream and charge' and fits well with this menagerie worlds Pratchett-esque attitude of 'people come in all shapes and sizes' and you never know from sight who might be friends or foes.
Another major 'house rule' that I am currently using is to port a mini-AD&D Spelljammer helm into 3.5e as a Spelljamming Saddle and also the Aces High sky combat rules from Arcadia magazine. An example of how that plays out in-game is here.
In addition, recognising that Stunning effects are no fun, I am thinking about a Revised Stun condition inspired by 2c Gamings post 'The Mysterious & Nonsensical Stunned Condition: Fixes & Critiques' that keep the "drops everything held, takes a –2 penalty to AC, and lose Dexterity bonus to AC" but dials it back from "can’t take actions" to only being able to do a single action or a move. No reactions, no 5' step, one thing on your round. Have not yet implemented this but thinking about it.
I thought about dropping in a Cascading Damage Dice - if you roll the max on a damage dice, roll it again. Keep going until you don't roll the max. This proved to be difficult to remember in course of play so has been dropped.
I am a big fan of 'Shields shall be splintered / Mail shall be rent' from Trollsmyth (I think?) where armour or a shield can be sacrificed (and the AC bonus lost) to soak the damage from one hit. However while the other rules above are more aligned with coming to a zone of high mythic weirdness, these are a bit more fundamental operating principles for the campaign and it does not feel right to try and bolt these in at a late stage in the campaign. Next campaign...