TrueCrypt is free, open-source disk encryption software.
It uses stronger encryption than FileVault (built into Mac OS X) and allows the encryption of entire volumes and partitions.
Make sure you choose the right download for your OS version and CPU type.
It's a good, simple-to-use encryption tool, but now it is replaced by VeraCrypt, which supports TrueCrypt vaults and will convert TrueCrypt to VeraCrypt format if required.