It's important to understand that if all you have is the id of any entity, then there is no way to load a specific entity with that id. Since entity_id's are not unique across entity-types, it is necessary to know both the entity type, and the entity id.
See Developers need an $entity->entity_type property for a discussion on changing this for D7, as well as info about how this will be likely be handled in D8.
On the other hand, as pointed out in a different answer, if you already know that the entity id is a node id, then you can call an entity specific load function like
node_load($nid). Since most entities will have such a load function to support named arguments, such a "shortcut" function will be almost universally available.
Examples: node_load(), taxonomy_term_load(), commerce_product_load().