The Entity Construction Kit (ECK) builds upon the entity system to create a flexible and extensible data modeling system both with a UI for site builders, and with useful abstractions (classes, plugins, etc) to help developers use entities with ease.
ECK allows the creation and management of entity types with custom properties; adding bundles to entity types; and fields to bundles, with the help of the Field UI module.
If that last sentence sounded like a foreign language, I suggest reading the first section of ECK's documentation: Understanding Entity Terminology