DeLong sends us to Miguel Almunia, Augustin S. Bénétrix, Barry Eichengreen, Kevin H. O’Rourke, and Gisela Rua: 18 November 2009:

The effectiveness of fiscal and monetary stimulus:

There is one important source of information on the effectiveness of monetary and fiscal stimulus in an environment of near-zero interest rates, dysfunctional banking systems and heightened risk aversion that has not been fully exploited: the 1930s.... [W]here fiscal policy was tried, it was effective.

...
Cross-country comparisons can thus help us untie the Gordian Knot and move the debate from the realm of ideology to that of evidence. Our project therefore focuses on assembling annual data on growth, budgets and central bank policy rates, mainly from League of Nations sources, for 27 countries covering the period 1925-39....

The details of the results differ, but the overall conclusions do not. They show that where fiscal policy was tried, it was effective. Our estimates of its short-run effects are at the upper end of those estimated recently with modern data; the multiplier is as large as 2 in the first year, before declining significantly in subsequent years....

The results for monetary policy are less robust but point in the same direction. A positive shock to the central bank discount rate leads to a fall in GDP... [that] just misses statistical significance at conventional levels.... This result is notable, given the presumption, widespread in the literature, that monetary policy is ineffective in near-zero-interest-rate (liquidity trap) conditions. On the contrary, in the 1930s it appears that accommodating monetary policy helped, by transforming deflationary expectations (Temin and Wigmore 1990) and by helping to mend broken banking systems (Bernanke and James 1991). Given the prevalence of both problems circa 2008, we suspect that the results carry over...

So ... fiscal policy gives more bang for the buck with a multiplier of 2 and monetary policy is less robust but pointing in the same direction. "Accomodating monetary policy helped, by transforming deflationary expectations and by helping to mend broken banking systems."