For the full thirteen year period since March 2000 (when the S&P500 reached the last of six annual new high watermarks going back to 1995) to March 2013 (when the index had firmly revisited the 1500 level) stocks have barely beaten the performance of the lowly money markets.
Had you invested $10,000 in, say, the Vanguard S&P500 Index Fund, VFINX, you would have reaped an extra $3,900.02 (39%).
But the same amount invested in Vanguard's Prime Money Market Fund, VMMXX, would have netted you $3,370.96 (33.7%).
Charts from Morningstar using Vanguard data:
