Reading Steiner recently 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.
Drama CD Alpha: Babel of the Grieved Maze
A story from Kurisu's perspective taking place before Okabe travels to the Beta worldline. There also is a translated manga adaption and an untranslated light novel.
Drama CD Beta: Arc Light of the Point of Infinity
Drama CD Gamma: Hyde of the Dark Dimension
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.
Comptiq Drama CDs
Some Drama CDs have not been translated yet, more details can be found on the Steins;Gate wiki.