I love the Mending spell; useful out of combat, sneakily important if you want a homunculus, and a spell no magic-user with an Intelligence under 16 would waste a slot on. I am half convinced that spell was put in the Player's handbook just for me.
But of all the magic-users you know, who ever takes it?
The tinker's hammer is a small tool hammer weighing less than 1/3 a pound. In desperate circumstances it could be used as a weapon, but things need to be pretty bad (speed 2, damage: 1 h.p.,, -2 to hit chain armors, -4 to hit plate armors, proficiency as club). It is, pure and simple a light tool.
Once per day the Tinker's hammer can cast a Mending spell as a 12th level magic-user. The Tinker's Hammer may only repair metal items. If used by a skilled person (in 1e someone with a secondary skill like blacksmith, in 2e the appropriate non-weapon proficiency) the effects of the Mending are increased (and DM's discretion) to repair greater damage or to even restore hit points to full plate armor.
If wielded by a spell caster that knows the Mending spell the Tinker's Hammer can be used an additional time each day by uttering the word 'damn' while using it. No Tinker's Hammer may be used more than twice in 24 hours.
Similar items (the Seamstress' needle, the Leatherman's Awl) exist to repair items not made of metal.