Some of Karlsruhe's nightclubs, from the perspective of a local:

Amadeus

According to its promotion, the largest indoor club in downtown KA.
Three dancefloors, the music ranging from cheese to trance to hiphop.
Worth checking out.
Tip: Don't like shelling out for the drinks? Swipe the bottles from behind the bar, the bar tenders will not notice if all of them have their backs turned or are serving at the other side!

Nachtflieger

Not what it used to be, however still promising during the week with a spacious interior and house / chillout / downbeat music.

Unverschämt

Decent metal / gothic club. Friday's recommended: the drinks are 1/2 price until 11pm. Free entry before 10:30pm.

Spa

Pleasant, social atmosphere. Pricey. Strippers on Fridays are to be recommended.

Nachtcafe

Avoid at all costs; consists of a largely aggessive, underage clientele who all dress identically. Cheap drinks.

Unterhaus

As above. Do not go, unless its on a thursday when the miniscular size of the place is forgotten about with €7 entrance for drinks on the house.