Mimiron is a brilliant inventor. His form is that of a mechanical gnome, though larger than normal gnomes. He is responsible for most of the mechanical creations in Ulduar and throughout Azeroth. Prior to his corruption by Yogg-Saron, Mimiron was a keeper of Ulduar and watched over the prison from his Temple of Invention in the Storm Peaks. The temple is now in disarray, with many of the mechanical beings there destroyed or malfunctioning. This is likely due to Loken's interference, and ultimately controlled by the Old God within Ulduar. Before his corruption, Mimiron created all sorts of things. Afterward he has worked solely on machines built for war.
Miimiron is currently located in the Spark of Imagination, a large workshop of sorts deep within the mountains behind Ulduar. In order to access it, adventurers must ride the LMS Mark II, a large tram created by Mimiron. Within the workshop, Mimiron is putting the final touches on his latest war machine. the V-07-TR-0N. No, I'm not kidding, that's what it's called.
V-07-TR-0N consists of 3 different machines. The base is battle tank called the Leviathan MKII and is similar to the Flame Leviathan that guards Ulduar's front gates. The body of the machine is the VX-001, an anti-personnel assault cannon that resembles two arms. The head of the machine is the Aerial Command Unit.
From quests in the Borean Tundra, it is speculated that gnomes were once mechanical beings created by a titan called "The Grand Architect" and that they evolved into beings of flesh through a natural process. Mmiron is not a titan, just a being empowered by them, so it is not known if he played a part in the creation of the race of gnomes. But it seems likely he did.
The Flame Leviathan was created by Mimiron to defend the outer gates of Ulduar, or perhaps to dissuade and protect treasure hunters from what is in side. However, with the prisoner now free, it is an obstacle that must be overcome if anyone is to get inside and free the keepers and put Yogg-Saron back in his place. The large tank is powered by four orbital towers that increase it's power when active. The four towers correspond the the four watchers within Ulduar.
XT resides in the scrapyard of Ulduar, and is presumably in charge of the dismantling of Mimiron's experiment's that have not worked. This is speculation based on his name, and his ability to pound the ground with his two large arms, deconstructing anything in between. Unlike the Flame Leviathan which is wholly a machine with standard programming, XT has been given the ability of self awareness. He considers himself the child of Mimiron, his creator, and thus talks and behaves as a young boy. He imagines the adventurers he comes across as his toys. He's bad about breaking them. And he's prone to throwing tantrums.
It's not clear what Mimiron had created before being influenced by Yogg-Saron, but logically it would seem that XT-002, and the Flame Leviathan have more basic purposes, and were created before the release of the Old God. The V-07-TR-0N however appears to be a new invention, it's sole purpose being destruction, and most likely made after Mimiron's primary directives where corrupted.