An amateur photographer captures an amazing photo of a kingbird riding atop a hawk....
Remora eels have a sucker for attaching to sharks. They ride the sharks, and scavenge leftover food and parasites on the shark.
Many birds ride about on the back of large mammals, picking off ticks.
The moth ear mite
My favorite story is about the moth ear mite. Bats hunt moths, using their sonar. Moths can hear the bat's sonar cries, and when they do, they spiral downward to avoid becoming a meal for bats. So having good hearing a important for the moth's survival.
Now the story thickens. There is a kind of mite that infests the ears of moths. When a moth visits a flower and sips the nectar, the moth ear mite is waiting there, and climbs up the moth's proboscus as it sips. The mite bushwacks back through the moth's scales, till it reaches one of the moth's ears, somewheres amidships. The ear cavity is like a cave for the tiny mite. The mite sets up home there, has a family, and, well... they make a mess of their new digs. The moth goes deaf in that ear.
Now what happens when the moth visits another flower? If there's another ear mite waiting there, where will it go? If it goes to the other ear, then the moth will be totally deaf. That's not good for either the moth or the ear mite, because the moth won't be able to avoid bats. The second ear mite (plus the moth and the mites in the other ear) are all going to be eaten by a bat if it goes to the good ear. So the second moth ear mite follows the path through the scales that the first mite left. It arrives at the same ear, and moves in. The moth and his colony of ear mites continue to avoid the bats. Isn't evolution wonderful? More.
Clams & other small aquatic critters
Since freshwater clams don't swim, and can only drift downstream, how can they colonize the upper reaches of a stream? They hitchhike, of course! Their tiny larvae bore into the gills of fish, which carry them upstream. Later, they drop out of the gills and live like a proper clam.
Many other small aquatic critters can hitchhike on the feet of ducks. The ducks sometimes have muddy feet, and the critters can ride in the mud. That's how they colonize new ponds far away. Air transport.