The E6 Ruleset is a set of variant rules for Dungeons and Dragons 3.5, with the intent of limiting the uneven power growth of characters and increasing prioritization of equipment. Under E6, character progression continues normally until level 6, where level gain stops. Instead, characters gain a bonus feat for every 5000 XP.
The intent of this ruleset is to prioritize tactical considerations and maintain a sense of risk, or at least cost, in even low-level encounters. No one is too powerful to be killed when asleep, even by a simple commoner. No magic stretches the logistics of the campaign setting beyond believability.
Note that the interpreation of E6 used by the UOGC Eberron campaign is slightly different than the official standpoint. While "standard" E6 assumes that sixth level is an epic level of power, this game's assumption is that no one is epic; sixth level is just the point of diminishing return at which experts plateau, even if most people lack the drive or ambition to reach it before old age (if at all).
Outgoing link: E6 documentation on RPG.net
Magic in E6Edit
Though E6 limits magical spells to that of third level or lower, characters can achieve higher magical effects through the use of incantations. Note that while incantations have the advantage of theoretically unlimited effects and power, they tend to be difficult, unreliable, time-consuming, expensive and situationally dependant affairs.
In the high-magic world of Eberron, incantations are not discovered, but developed. Arcane theorists of sufficient knowledge and talent can design tailor-made incantations for specific ends. However, this is an extremely research-dependant process (and arcane theorists don't come cheap), so it may be easier to research some patented rituals instead.
The only magical items in the setting are those of CL 8 (the effective caster level of a sixth-level Artificer) or lower. Exceptions are vanishingly rare and limited to ancient artifacts or divine creations. While creative design can allow for some seemingly epic devices (Sharn's sky coaches and Skyway are combinations of relatively low-level spells and ingenius "mundane" engineering), PCs should remember that their equipment list is much less important than in vanilla D&D.