Off the top of my head, using only the minimal length examples:
bib, bob
dad, deed, did, dud
eke, ere, eve, ewe, eye
gag, gig
kook
mom
noon, nun
pap, pep, pip, pop, pup
radar
sees
tat, tit, tot
wow
And, of course, there is one Scrabble-legal letter combination that is two letters long and a palindrome: AA. A'a (pronounced /ah ah/) is a type of lava, which is thicker and more viscous than other types. It is characterized by flowing in a sporadic fashion and leaving a rough surface when it cools.
And if it's not already been suggested, then I propose a variation of Appletters called Palindrominoes: The word snake is formed as in regular Appleletters, but if you can form a palindrome where the base letter (the "head" of the snake, I suppose) is inside the palindrome, then you are permitted to position letters both above and below the end-tile, as shown in the example below:
eThen you can give the snake multiple heads, like a hydra or a planarian or something. A little extra chaos to spice up your Appletters game.
VIGOR
e I
N
KITE