A guild that was only mentioned once in the series, in Jingo. When Vetinari was telling the city's leaders about the war, they asked how the weapons were doing. The Guild President remains, at the publication of The Compleat Ankh-Morpork, Mr Burleigh of Burleigh & Stronginthearm (who may also be the Burleigh of Burleigh and Spoke, a company manufacturing civilian spin-offs from armour technology).
The Guild of Armourers are based at The Armoury, on Siege Street*(F5), on the Isle of Gods. This may be the same Armoury featured as a location in Men at Arms, when Carrot needed to re-arm the Watch and arm the Watch Militia. In which case they provided, pro bono and for the good of the City, Detritus' formidable Piecemaker.
* The Compleat Ankh-Morpork map coordinates.