I have used Fidelity for over 10 years. They have a wide variety of mutual funds to choose from and their online interface is intuitive and easy to understand. Best of all, they have a LOT of low-cost mutual fund options.

Vanguard is another industry leader that I trust because of the reputation it has developed. This company is completely geared toward helping its investors succeed in their retirement planning by providing simple, low-cost retirement solutions like index funds.

Here are some helpful articles to get you started with investing for your retirement:

Roth IRA Investing in 10 Simple Steps
Smart Retirement Investing the Vanguard Way