The Raspberry Pi (RPi) is an amazing computing platform; it's small, it's cheap, it's fast, and it's remarkably versatile. So versatile in fact that with practically no additional hardware you can turn one into an FM transmitter …
What you'll need is an Raspberry Pi 1 or 2 running Raspbian, and, optionally, a short length of wire as an antenna. For software you'll need one of the following:
PiFm - written in 2012 by Oliver Mattos and Oskar Weigl of the Imperial College Robotics Society PiFm consists of a Python program that loads an executable written in C which does the heavy lifting. Can transmit both mono and stereo audio. Works on all Raspberry Pi 1s.
fm_transmitter - written by Marcin Kondej in July 2015, is completely written in C and works on both the Raspberry Pi 1 and Raspberry Pi 2. The sound quality is apparently not as good as the PiFm-based programs and while it can read both mono and stereo audio files can only transmit mono.
According to the PiFm documentation, the RPi FM transmitter is fairly powerful delivering a signal that can be detected at ranges of 50 meters or more. The RF signal is output on GPIO 4 which is pin 7 on both the 26-pin header on the Raspberry Pi 1A and 1B, and the 40-pin header on the Raspberry 1A+, 1B+ and 2B boards.