1. If you think the appraisal is correct, you should request the seller lower the price, provided of course you have an appraisal contingency.
2. If its an FHA appraisal, that price is stuck on the house for 6 months for anyone using an FHA loan - nothing you can do here except pay the difference between the contract price and the appraised value in cash. The lender will only lend up to the appraised value.
3. VA loan - you can rebut a VA appraisal through the tidewater act. You and your agent will have an opportunity to submit why you think the property is worth what you offered on it. Whether you win this battle or not will all depend on how well you write up your rebuttal. The VA appraised price will also stick on the house for 6 months for anyone using a VA loan.
4. If its a conventional loan, you can switch lenders and order another appraisal to see what value you get.
Hope that helps. If you really want the house and need to talk about more options, feel free to email me any time and I will do what I can to help.