Zooxanthellae and corals/anemones.
As an aside, symbiosis is actually a broad term for interactions between organisms of different species. Symbiosis can fall into several classes. Commensalism where one benefits and the other is unharmed. Parasitism where one benefits and the other is harmed. And mutualism where both benefit. I think that's cool!
I'd also like to try a clingfish/long spine urchin combo.
Also do adult Bangaii Cardinals typically stay in urchins, or is this only seen with younger fish?
Are these the clingfish you're talking about?
http://www.liveaquaria.com/product/prod_display.cfm?c=15+31+1483&pcatid=1483
Diademichthys clingfish are not necessarily as closely tied to Diadema urchins as the name implies. I have seen these guys in the wild around 20-30' away from the nearest Diadema urchin. When startled they don't appear to flee back to them either, they just hunker down in branching coral like most other fish.
the emperor shrimp and medusa worm (sea cucumber) or the pearl fish and sea cucumber.