Ni Karma (NKS)
Although I would like to present these loot systems in an objective method, I cannot help but mention that this is perhaps the most ingenious loot system that I've come across in my days of playing WoW. Ni Karma, or the Ni Karma System (NKS), was developed by one of the many guilds named Knights Who Say Ni and is a pseudo-zero-sum DKP system that operates on, as the name implies, karma. NKS should not be confused with a DKP system, however, because even though players accrue karma points as they might DKP points, you cannot explicitly spend karma in order to gain an item. Instead, NKS is more akin to an open roll system in which players may adjust their rolls through the usage of karma -- essentially adding in the equality nature that is missing from a straight open roll system.
Within NKS, karma is awarded to players for various things that are dependent upon the guild, as you might gain DKP. Karma may be awarded for showing up on time, killing bosses, time spent within a raid, work on a progression kill or whatever other variables a guild may choose to use. Once an item drops, it is then placed up for bid within an open roll system. Before rolling, each player is given three options: pass, roll without karma or roll with karma. After the selections are made, the rolls are automatically produced and the player with the highest roll total wins. Should a player choose to roll without karma, he gains a standard /roll, as in a traditional open roll system; players who choose to roll with karma have their current karma total added to their roll result to create a new total.
Thus, if player A chooses to roll without karma and ends up with a 49, then his roll total would simply be a 49. However, if player B chooses to roll with karma, then his current karma total is added to that value; if he has 25 karma and rolls a 25, his roll total would then be 50 and he would win the item. Players can only choose to roll with or without their karma, and they can only make that choice before they actually roll. When a player chooses to roll with karma, he uses his entire current karma total -- using a portion of karma is not allowed. A player who chooses to roll with karma and wins an item then has his karma reduced by 50%, rounded up to the nearest factor of 5. Note that the value is always rounded up; therefore, all values of 1-4 are rounded up to 5, and all values of 6-9 are rounded up to 0. Players who choose to roll with karma but do not win the item suffer no penalty.
Should a player who chooses to roll with karma have 50 more karma points than anyone else choosing to roll (with or without karma), then he automatically wins the item without a roll taking place and is docked the normal amount. This condition is put into place to create a more equal system in which players with exceptionally bad luck rolling don't constantly lose to players with exceptionally good luck rolling. (For example, spending 50 karma and rolling a 1 while someone without any karma merely rolls a 52 would never happen within the system.)
The creators of the system note that it's important to factor in how much karma is entering and exiting the system at any given time. You do not want so little karma that long-time members feel they have no advantage over newer raiders, while conversely, you don't want a massive build-up of karma points that would prevent newer members for ever getting loot. The suggested rate should be approximately 20 karma earned per raid; however, the actual value should be determined by the guild itself.
Alternatively, guilds can add a minimum karma cost to all items, should karma hoarding ever become an issue, To this end, every time a player wins an item without using karma, he is still docked a nominal amount of karma points in order to prevent "roll fishing" (in which a player hoards karma for a specific item but still attempts to snag other pieces of loot that players who are using their karma may want). This, however, is one the wonderful things about the system, as it can act as both a DKP variant and an open roll variant at the same time. Stockpiling karma for a specific item allows karma to function more akin to DKP -- in which the player reaches the 50-karma point and is then able to "purchase" the item through raw karma, but at a very high karma cost -- or a player can use karma as merely a roll bonus at low levels of karma, to increase his chances of success but at a relatively low karma cost.
Shroud Loot System (SLS)
Last but certainly not least is Shroud Loot System, or SLS. SLS is a form of DKP-variant system that is somewhat similar to Ni Karma but so very different. Within SLS, players earn DKP as they normally would; however, bidding is done based upon two principles. Each item that drops has two different cost plateaus; a player can either choose to pay a low, fixed price amount for an item, or he can choose to spend 50% of his current total on the item. There is no actual bidding within Shroud. A player can only choose one of those two options; he cannot choose a higher or lower amount to spend.
In the event that two players tie in the amount of DKP that they are willing to spend (which generally only occurs when players are only willing to spend the lower, fixed cost), then a random roll is used to determine the winner. The brilliant principle that drives SLS is that it isn't based on an auction-style bidding system, as traditional DKP systems are. In this manner, bidding is much more streamlined, and DKP hoarding is prohibited, as similar to Ni Karma, players will always have to spend at least 50% of their current DKP to gain an item that they are seeking. Through this, a hoarder is going to find his stash depleted rather quickly with only minimal gains, which tends to balance out the system naturally instead of requiring a need to adjust DKP values. Having the fixed price on items is a brilliant plan for the more minor upgrades or players getting items for their off spec; it allows them to not lose a significant value of DKP on a low-priority item, but it prevents them from getting loot for nothing, which can lead to inflation. (Conversely, however, high-cost off spec items generally keep hybrid classes from gearing up their secondary roles, which can hurt a guild in the long run.)