One of 2008's most discussed articles in positive psychology circles was Dunn, Aknin, & Norton's "Spending money on others promotes happiness" published in the journal Science. Brieflly, the authors examined the relationship between happiness and spending, and focused on differences between spending on self versus spending on others.
The article described three studies. One was a national survey. Another was a study of "windfall spending" (employees of a Boston medical supply company were given bonuses; spending was tracked and relationships to reported happiness were observed). The final study involved subjects randomly assigned to spend money on either themselves or others.
In this third study, the authors reported that "very minor alterations in spending allocations....may be sufficient to produce non-trivial gains in happiness on a given day."
Here's a .pdf of the article: http://research.chicagogsb.edu/cdr/docs/spendingmoney-norton.pdf
And here's an article about the study in the Boston Globe: http://www.boston.com/business/personalfinance/articles/2008/03/21/money_makes_you_happy___if_you_spend_it_on_others/