Familiars... lessee.
In one game, had a awakened dire badger sorceror -- his familiar was his badger son. It added the joy of having a familiar to the problems of parenting a teenager. He was a gond follower, and the wild mage in the party tended to experiment on the familiar -- at one point the familiar was granted an intelligence of 29 for about ten minutes -- the sorceror was a Gond follower, so they whipped up the specifications for a rifle in the time his brain held out -- afterwards the familiar had an inadequacy complex. It was a fun story, wouldn't have been the same without the familiar being the son of the PC. (And there was also the backstory of rescuing the kid from the evil ex-wife they both hated, but the game didn't last that long.... for... some.. reason or another...)
The other familiar story I delt with lately had the mystic theurge in the party, who started as a cleric, approached by a bird, who was sure it was not a bird, who knew it was his familiar. The barbarian in the party tried to turn it into a mouse, but ended up with an undead bat with skeletal wings, the furry body of a mouse, the head of a bat, and a single skeletal bird leg dangling from its body. Bert (not a bird and not a bat) was primarily used only as a story device of self pity, and an example of what the barbarian would do to people if they were bad.
In short, all of my fun stuff with familiars has been with stuff that was completely off the wall.
I've mainly had a lot of fun with familiars that like their master, but generally consider themselves superior on some level, and tend to be smartalecky... fun that way for me to DM.
In Changeling I had a pseudodragon that wore a dinner jacket and a monocle, had a British accent, and did my homework for me, but that's a different game... fun though.