"What it does: Sells natural gas and electricity. The company's most popular offerings are fixed-price one- to three-year contracts tailored for homeowners and small businesses. Why it's growing: Volatile natural gas prices have driven homeowners and entrepreneurs to seek the predictability that MXenergy's long-term contracts provide. The bankroll: MXenergy has raised several rounds of capital, including Harvard endowment money and, in 2004, private equity totaling $31 million."