The literal answer to your question is that a number of different types of mutual funds did not have significant downturns in 2008. Money Market Funds are intended to always preserve capital. VMMXX made 2.77% in 2008. It was a major scandal broke the buck, that its holders took a 3% loss.
Inverse funds, which go up when the market goes down, obviously did well that year (RYARX), but if you have a low risk tolerance, that's obviously not what you're looking for. (and they have other problems as well when held long-term)
But you're a 24-year-old talking about your retirement funds, you should have a much longer time horizon, at least 30 years. Over a period that long, stocks have never had negative real (inflation-adjusted) returns, dating back at least to the civil war. If you look at the charts here or here, you can see that despite the risk in any individual year, as the period grows longer, the average return for the period gets tighter and tighter. If you look at the second graph here, you see that 2011 was the first time since the civil war that the trailing 30-year return on t-bills exceeded that for stocks, and 1981-2011 was period that saw bond yields drop almost continuously, leading to steady rise in bond prices.
Although past performance is no guarantee of future results, everything we've seen historically suggests that the risk of a broad stock-market portfolio held for 30 years is not that large, and it should make up the bulk of your holdings. For example, Vanguard's Target retirement 2055 fund is 90% in stocks (US + international), and only 10% in bonds.