Not yet compatible with EE2.6+!
From the start, one of the big attractions of ExpressionEngine has been the ability to add unlimited custom fields to an unlimited variety of channels (formerly known as weblogs). It’s vital to be able to relate these various channels to each other, and this is done with relationship fields — and extended by such 3rd-party add-ons as the beloved Playa.
But even the grace and power of Playa is stymied by the limited way its relationships can be displayed on EE templates; while a channel’s related entries can be displayed, those related in turn to its related entries cannot; that is, Playa’s children, parents and siblings tags cannot be nested — nor can the native EE related_entries and reverse_related_entries tags.
Tied Entries overcomes this limitation, enabling unlimited traversal across a site’s various channels via their relationship and Playa fields. It’s like having a fully relational database for your channels.
While there are other ways for the undeterred EE developer to achieve this here and there in templates, either with direct MySQL queries or with a series of embedded templates (as Pixel & Tonic suggests), the Tied Entries plugin is more powerful than the former and more flexible and less resource-consuming than the latter. Plus it’s much easier — increasingly so as the number of relationships grows.
The plugin can also be used as a substitute for the standard related_entries and reverse_related tags when displaying entries just one relationship away because, unlike those tags, results can be filtered by categories and custom fields.
Tied Entries is compatible with both EEv1.x and EEv2.x. The EE2 version is fully compatible with Playa 4; the EE1 version works with Playa 3 and older but only passively, that is, in reverse relationships.