Mary Gentle's book "Ash: A Secret History" has all manner of nasty and magical machines of war, including armoured war wagons and golems with flamethrowers. If you've got a few weeks give it a read (it is a BIG book.)
Another source for war machines would be any of the books looking at WWII research into unusual weapons - there was a tv documentary many years ago called "The Secret War".
How about:
The Goblin Pinwheel Juggernaut
A huge cart wheel, over ten feet in height. The wheel's rim is made of two inch thick iron plates and is two - three foot wide. Mounted along the edges of the rim are a series of alchemical rockets. The entire device weighs over a tonne.
The goblin operators point the Juggernaut in roughly the direction which they wish it to go - using beasts of burden usually. They then ignite the rockets, which burn for 1 minute (ten combat rounds). The Juggernaut moves forward at the rate of 80' per round. It deals 10d6 damage as an area effect attack, plus a strength 35 Bull Rush effect to everything in the five feet squares through which it passes; Reflex save DC17 for half damage. Armour is no protection against this attack but evasion and improved evasion can apply. The Juggernaut's path cannot be stopped by anything of less than Huge size. If a Huge or larger creature (or object) successfully opposes the Bull Rush effect, then the Juggernaught is stopped for that round. (see below for next round)
Being of goblin manufacture, the rockets do not all burn at the same rate. Consequently, some burn out early or late, causing the weapon to behave erratically. At the end of each round of movement roll on the grenade scatter diagram to determine the new direction of the Juggernaught. In the next round the device will travel on the line of the new direction. If the Juggernaught was stopped in this round, but the rockets have not yet burnt out, roll the scatter and the pinwheel resumes its course. If the ten rounds are up, then the Juggernaught continues to move for one more round then rolls to a halt under its own weight.
The Juggernaught can be attacked during its course, using prepared actions or attacks of opportunity. It has an AC of 12 (10, -2 for size[Huge], +4 natural [erratic motion]), a hardness of 10 and 150 HP. A character who rolls a natural 1 when attacking the Juggernaught has accidentally struck one of the rockets, suffering the effects of a 5d6 fireball as it explodes (DC 15).
Anyhoo, how's that for an idea? More as and if they come to me.