With the stock market in its downward spiral, money market funds are an attractive option for many investors right now. But not all money market funds are created equal.

The most important factor in choosing a money market fund is its expense ratio, which can eat away at a fund's return since most money market funds generally have comparable yields. While the average expense ratio for money market funds is currently around 0.5%, many fund companies offer fees that are much lower than that, and some even waive their expenses to attract new customers.

One of the most up-to-date sources for money market yields is

iMoneyNet , which which ranks the top-yielding government, nongovernment and tax-free money funds for retail and institutional customers.