"The Great Slump of 1930" (Dec., 1930)
"The World's Economic Outlook" (May, 1932)
"Open Letter to President Roosevelt" (Dec., 1933)
The latter has some advice which should also apply to President-Elect Obama:
You remain for me the ruler whose general outlook and attitude to the tasks of government are the most sympathetic in the world. You are the only one who sees the necessity of a profound change of methods and is attempting it without intolerance, tyranny or destruction. You are feeling your way by trial and error, and are felt to be, as you should be, entirely uncommitted in your own person to the details of a particular technique. In my country, as in your own, your position remains singularly untouched by criticism of this or the other detail. Our hope and our faith are based on broader considerations.
If you were to ask me what I would suggest in concrete terms for the immediate future, I would reply thus.
In the field of domestic policy, I put in the forefront, for the reasons given above, a large volume of loan-expenditures under government auspices. It is beyond my province to choose particular objects of expenditure. But preference should be given to those which can be made to mature quickly on a large scale, as for example the rehabilitation of the physical condition of the railroads. The object is to start the ball rolling. The United States is ready to roll towards prosperity, if a good hard shove can be given in the next six months.
The 1930 essay comes to my attention via Paul Krugman, who writes "its observations on the crisis at hand remain stunningly insightful, and fit current events all too well."
Update (12/5): Keynes' private letter to FDR (Feb. 1938). This passage is particularly amusing:
Businessmen have a different set of delusions from politicians, and need, therefore, different handling. They are, however, much milder than politicians, at the same time allured and terrified by the glare of publicity, easily persuaded to be ‘patriots’, perplexed, bemused, indeed terrified, yet only too anxious to take a cheerful view, vain perhaps but very unsure of themselves, pathetically responsive to a kind word. You cold do anything you liked with them, if you would treat them (even the big ones), not as wolves or tigers, but as domestic animals by nature, even though they have been badly brought up and not trained as you would wish. It is a mistake to think that they are more immoral than politicians. If you work them into the surly, obstinate, terrified mood, of which domestic animals, wrongly handled, are so capable, the nation’s burdens will not get carried to market; and in the end public opinion will veer their way.