What do you call a person who doesn't/can't usually keep his/her word ?
A liar if you want a simple term.
If it is someone who regularly makes promises and doesn't live up to them they are a renegger.
You may also see a moderately offensive (to some) term Indian Giver which is a reference to Native Americans. This term was very popular before the PC movement.
And if it is someone who doesn't pay their bets they are a welcher.
I would usually call them a Flake.
Per David M's Request:
n. An unreliable person; someone who agrees to do something, but never follows through.
1. John called in sick to work again today. He's such a flake.
2. Mary said she would do the research for our project, but it's been a week and she hasn't done a thing. She's such a flake.
-5. slang chiefly ( US ) an eccentric, crazy, or unreliable person
Such a person is 'untrustworthy'
A "welcher" is someone who backs out of a commitment. Since it's origin may refer to the Welsh people, this may be considered offensive.
Per Dictionary.com, from the verb "welsh" or "welch":
verb (used without object)
Informal: Sometimes Offensive.
to cheat by failing to pay a gambling debt: You aren't going to welsh on me, are you?
to go back on one's word: He welshed on his promise to help in the campaign.
Origin: 1855–60; perhaps special use of Welsh
A promise-breaker would probably have the clearest meaning of them all, but it's not the 'one word' you might be looking for. Reneger is a good alternative.