S&P 500 is a widely recognized stock index, that many people benchmark performance against, and you can find 'passive management' funds that compete to replicate it at as low a fee as possible.
But investment portfolios shouldn't be comprised of just stocks. Is there a widely recognized version of S&P 500, but for bonds?
Multiple overlapping indices exist covering various investment universes. Almost all of the widely followed indices were originally created by Lehman Brothers and are now maintained by Barclays. The broadest U.S. dollar based bond index is known as the Universal. The Aggregate (often abbreviated Agg), which is historically the most popular index, more or less includes all bonds in the Universal rated investment grade. The direct analog to the S&P 500 would be the U.S. Corporate Investment Grade index, which is tracked by the ETF LQD, and contains exactly what it sounds like.
Citigroup (formerly Salomon Brothers) also has a competitor index to the Aggregate known as Broad Investment Grade (BIG), and Merrill Lynch (now Bank of America) has the Domestic Master.
Multiple other indices also exist covering other bond markets, such as international (non-USD) bonds, tax-exempts (municipal bonds), securitized products, floating rate, etc.
The iShares Barclays Aggregate Bond - ticker AGG, is a ETF that may fit the bill for you. It's an intermediate term fund with annual expenses of .20%. It "seeks investment results that correspond generally to the price and yield performance, before fees and expenses, of the Barclays Capital U.S. Aggregate Bond Index"
Keep in mind that the bond market is dominated by US Treasury securities... if there were an S&P 500 for bonds, the US would take positions 1-400. Be careful that you understand what's in your bond funds -- you may not be as diversified as you think.
