This is something I see mentioned a lot, and I think it's important to distinguish that the program can be evaluated for free, but you really should purchase a license.
Originally Posted by fiziwig
From their site:
REAPER is available for download without technological limitations for evaluation purposes. Once you have evaluated REAPER, you should purchase a license:
Purchase non-commerial license for $39.95 USD plus tax (via RegNow)
Purchase commerial license for $199.95 USD plus tax (via RegNow)