According to the FCC, you can use absolutely any frequency you wish at less than 0.1 watts without a license. There are also specific frequencies that you can use without a license such as 915 megahertz, 2.4 gigahertz, 5 gigahertz, etc. You can also use something like GMRS, which while requiring a license does not require any kind of test, but just a fee for the license which lasts for like 10 years.