All recent Linux distributions contain a free implementation of iconv utility and iconv() function as part of the GNU C Library which is the C library for current Linux systems. The library's iconv() function is licensed as LGPL, so it is linkable with closed source applications, while the iconv utility is licensed as GPL, so derivatives mush be open and in GPL too. To use the library, the GNU glibc locales need to be installed, which are provided as a separate package (usually named glibc-locale) normally installed by default.
Under Microsoft Windows, the iconv binary (and thus, likely also the API) is provided by the Cygwin and GnuWin32 environments or native Win32 port win_iconv.exe.
iconv is also available for many programming languages. For example, one of the libraries supported by PHP (also under Windows using a DLL file), so it is possible to use iconv() from a PHP program easily.