I have two I will be using in my next campaign:
Ring of attunement: Provides 1 extra attunement slot. (Requires attunement)
Event Staff: This staff allows the wielder to gain unquestioned entry into any “employees only” areas or zones otherwise off-limits to the public. Anyone (including actual staff or other officials) who sees the wielder in one of these areas will assume they are a known employee or other official who is granted special access to the area. Unfortunately, they will all also view the wielder as the least competent and least trustworthy employee or official with the organization. Any actions taken in the area are likely to be closely watched and highly scrutinized by any observer who would know better.
ETA: One from the current campaign in which I am a player character. Our DM thought of this one:
Bullet of Healing:
This magical bullet can be loaded into any firearm. Whomever is shot by this bullet first receives 1d6 piercing damage followed by 1d10 healing. If the initial damage causes recipient’s HP to fall below 0 before the bullet’s healing effects begin, they will fall unconscious and will not gain any healing effect from the bullet. Instead, one death save is automatically passed.
I played a campaign where we had a dagger of healing. It worked great against undead (as intended by the DM) and also to torture information out of people (not intended by DM).
That bullet of healing is awesome. Would be great for a supernatural Wild West game. Which, is I think what I want to run next now.
The Ring of Attunement idea has come up before, and if I remember correctly there’s a class (Artificer?) that gets a bonus based on how many items they have attuned, making it a genuinely useful item in niche cases.