Why Land Raider conversions are so popular? Because they are MASSIVE! So, you have a lot of space for your experiments. As for Chaos Land Raiders, you have even more freedom - the mutation of the Warp can be really impressive. So, look at the most unusual conversions found in the Internet.
The standard Land Raider, also known as the Land Raider Phobos, is an Imperial main battle tank and troop transport which serves as the "armoured fist of the Space Marines." Available to the Space Marines, the Traitor Legions of the Chaos Space Marines and the forces of the Inquisition, it is one of the most resilient and iconic vehicles in the Warhammer 40,000 universe. The Land Raider's heritage predates even the founding of the Imperium of Man, yet it remains the single most destructive weapon in the Adeptus Astartes' arsenal.
Protected by bonded Ceramite and Adamantium armour, the Land Raider is nearly impervious to all save the most destructive weaponry. Equally impressive are the Land Raider's armaments: 2 Twin-linked Lascannons and a Twin-linked Heavy Bolter allow the Land Raider to deliver punishing fire support capable of decimating enemy infantry and armour alike.
The history of the Land Raider is a long and complicated tale. The original designs for both the armour used in the Land Raider and the anti-gravitic technology used by the Land Speeder were discovered during an expedition deep into the Librarius Omnis on Mars by technoarchaeologist Arkhan Land, after whom the vehicles were named. They were once used by all the various branches of the Imperium of Man's armed forces, but during the Horus Heresy, their use was restricted to the Space Marine Legions by decree of the Emperor of Mankind himself.
The popularity of this mighty war engine was so great, that a whole Forge World, Anvilus 9, was entirely turned over to Land Raider production. Anvilus 9 was overrun by the Traitor Tech-priests of the Dark Mechanicum at the beginning of the Horus Heresy, and Land Raider production suddenly slowed to a trickle. With Horus' forces threatening to overrun Terra, the Emperor ordered that all Land Raiders still in Loyalist hands were to be handed over for exclusive use by the Space Marines, who were at the forefront of the fighting.
After the Heresy, the decree of exclusive use by the Astartes remained in place. The decree has remained uncontested for ten thousand standard years, although a small number of Land Raiders have been manufactured for use by the Ordos of the Inquisition.
The standard configuration Land Raider, known as the Land Raider Phobos Pattern, is one of the most powerful main battle tanks in the Imperium's arsenal, armed with two twin-linked "Godhammer-pattern" Lascannons mounted in side sponsons. A twin-linked Heavy Bolter is mounted on top of the tank to provide anti-infantry fire. As with most Space Marine vehicles, the Land Raider can also be armed with a pintle-mounted Storm Bolter and a single-shot Hunter-Killer Missile Launcher. The Land Raider is supposedly the most heavily armoured vehicle available to any army smaller than a full Imperial Guard armoured company, and its armour is believed to be second only to that of the Necron Monolith in strength.
Particularly noteworthy is that the Land Raider's all-around armour is equally thick everywhere on the vehicle and has no weaknesses, unlike other tanks that often have thinner side and rear armour. Its durable protection, combined with its transport capacity (10 Space Marines in Power Armour or 5 Terminators), make it essentially a mobile, fortified bunker, save for the lack of firing ports for the passengers. Land Raiders are also equipped with a "Machine Spirit", an artificial intelligence crafted by the Adeptus Mechanicus that allows the hulking behemoth to move and fight, even without a crew, as if the vehicle were on autopilot.
The Chaos Space Marines also have access to these powerful machines as they took with them countless numbers of these destructive behemoths following their defeat on Terra during the Horus Heresy. In the Eye of Terror, the Machine Spirits of the Chaos Land Raiders used by the Traitor Legions were destroyed and replaced with Infernal Devices containing the captured spirit of a daemon that performs similar functions for the vehicle.
If you know about other cool conversions, feel free to post them in the comments below. And fine terrain for such models can be bought here