I know restricted and multiverse are not free software, but what about source code and independent?
EDIT : I'm referring to free as in freedom.
Main provides free software (as in freedom) that is supported by Canonical.
Universe provides free software (as in freedom) that is supported by the community.
Restricted provides non-free software that is supported by Canonical. (This consists mostly of proprietary drivers for officially supported devices.)
Multiverse provides non-free software that is supported by the community.
The Partner repository, practically speaking, provides non-free software. The Extras repository provides some free software but mostly provides non-free software.
Many repositories provide both binaries and source code. These are enabled separately in your APT configuration, however. Both binaries and source code from Main are free (as in freedom), for example.
There are no repositories in Ubuntu that are actually called Source code or Independent. Perhaps some other OS has repositories called that. Or perhaps those terms appear in descriptions somewhere for some of Ubuntu's repositories.
See Repositories/Ubuntu for more information.