All about Steins;Gate

Besides the music from the anime, visual novels and movie, there are also bonus CDs and the beautiful Symphonic Reunion.
A compilation of all Science Adventure music is listed very nicely on the /r/steinsgate wiki.

Reading Steiner has translated the character songs, which were released in 2010.

The drama CDs are short audio-only stories recorded with the same voice actors as in the anime and VN.
Some were subtitled by Reading Steiner and uploaded to his Youtube channel, but it was taken down. They have however been reuploaded by JD2390 or can be downloaded from Link3693's mediafire.
A useful document was put together by /u/Brickorblock containing synopses and more detailed information about all Drama CDs.

A story from Kurisu's perspective taking place before Okabe travels to the Beta world line. There also is a translated manga adaption and an untranslated light novel.

A story from Mayuri's perspective taking place at the end of the anime and visual novel. The manga adaption is the process of being translated.
It has been partly translated and animated to look like the visual novel by Sarkkoth on YouTube.

A story from Okabe's perspective in the gamma attractor field, where he finds himself in a difficult situation. This is currently the only title that takes place in the Gamma Attractor Field.

These Drama CDs take place before and after the main events of the movie. Only the second one has been fully translated.

This series of Drama CDs was included with a limited edition release of the movie, and takes place in 2011.

The Epigraph trilogy light novels were accompanied by a drama CD consisting of three parts: Epitaxy of the Projected Curved Surface, Pandemia of Artificial Machinery, and Arpeggio of Time Rotation.
It has been partly translated and animated to look like the visual novel by Sarkkoth on YouTube.

Two slice of life Drama CDs, the first about Okabe who has fallen ill and the second about going to the beach.

A Drama CD about the round table meetings in the lab which was included with a limited edition of the original manga. It has been fully translated, be sure to enable the subtitles.

In the first show, Kurisu is blackmailed into participating in a new radio show segment. Mayuri and Fairis attempt to convince Kurisu to dress up in a maid outfit, nya.
In the second show, a while later, the radio show is revived in order to obtain funds after using it all up. It includes a new segment: a special quiz show.

Several Drama CDs were released during and after the release of Steins;Gate 0.

There are a bunch more Drama CDs that have been translated, but I couldn't find any short descriptions of them.

Some Drama CDs have not been translated yet, more details can be found on the Steins;Gate wiki.