For many investors, 2013 has been a good year.

On May 7, the Dow Jones industrial average broke 15,000 for the first time ever. Since Jan. 1, the S&P 500 is up more than 15%. Microsoft, a company whose share price has been relatively flat for years, is up more than 30%.

But there is one market sector that never got invited to the party. In fact, many of the companies that make up this unique market niche have seen their share prices plummet over the past three months, falling by as much as 20%.