The slightly longer list of the 106 permissible two-letter words for British tournament Scrabble use, drawn (but not cut and pasted) from the Chambers dictionary: definitions given for words not in the American list. These are rather more important in cutthroat competition (whether it be in tournaments or against your SO's grandma) than Jurph suggests, since they enable new words to be played alongside others to create multiple scoring opportunities:
bo - the sort of tree under which Buddha found enlightenment, or US slang for a bloke
ch - south western English dialect form of 1st person subject pronoun
da - father (dialect) and a Burmese knife
di - plural of deus
ea - river or drainage channel
ee - eye (Scottish)
fy - = fie, expression of disapproval
gi - what you wear to do judo
gu - a type of viol played in Shetland
io - a cry of invocation, joy or despair (in various standard translations of Greek drama, I think)
ko - a Maori digging-stick
ky - a cow (Scots, the singular of kine)
ny - variant spelling of nigh
ob - an objection (archaic lawyer's slang)
oi - interjection as in "oi, you, why are you playing that poncy game?"
oo - Scots dialect forms of "wool" and "we"
ou - Scottish interjection expressing consent (variant of "och")
st - a noise you make to attract someone's attention
te - musical note in the tonic sol-fa (same as "ti")
ug - to arouse or feel loathing
ur - um, er, ...
yu - a form of jade
zo - a cross between a yak and a domestic cow, also "zho".
Words found in the American list but not included in the British one: ab, ag, ed, et, hm, mm, pe, uh, xu, ya. Note that in international tournament Scrabble in English, the official British and American lists are combined.
No indication of comparative merit is intended for this list, and my views on playing Scrabble on a never-mind-the-meaning-just-look-at-the-points basis concur largely with dannye's here. This list is drawn from the second edition of Official Scrabble Words; I think there is a newer version based on the current edition of the Chambers dictionary, which certainly has an entry for xu - /msg me if you know of updates. And remember, there are no two letter words with a V in them.