All recent Linux distributions contain a free implementation of iconv() as part of the GNU C Library which is the C library for current Linux systems. To use it, 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.