Yes. You can contribute to a Roth IRA or Traditional IRA regardless of whether or not you have an employer-sponsored plan. While participation in a retirement plan does not change how much you can contribute to an IRA, it can affect whether or not you are eligible to deduct your Traditional IRA contributions on your tax return.
IRS Circular 230 Notice: The information contained in this FAQ is provided for information purposes only, is not intended to constitute tax advice which may be relied upon to avoid penalties under any federal, state, local or other tax statutes or regulations and does not resolve any tax issues in your favor. Refer to IRS Publication 590, Individual Retirement Accounts for additional information on IRAs in general and consult your tax advisor about your individual tax situation.