Vanguard. Large company that offers a vast line of mutual funds and ETFs with low maintenance costs. Specialized in index investing. They provide brokerage, for non-Vanguard instruments too, and other value-added services for retirement, college and institutional savings.
Fidelity. Another large fund company suitable for the buy & hold investor, with a similar profile and product line as Vanguard.
T. Rowe Price. Very-well regarded mutual fund company, providing a complete line of low-maintenance-cost funds for individuals and institutions.
iShares. Big players in the exchange-traded fund (ETF) market, by Barclays Global Investors. Through their multi-lingual site, they provide information about their extensive line of broad and specialized ETFs.
The Royce Funds. Small-cap fund provider, open and closed end. They carry an assorted line of small-cap funds with different focuses.