I evaluated a lot of different open-source course registration systems available on the internet. Unfortunately, most of these systems offer a lot of functionality, but are not very easy to use and integrate. The aim was to keep the maintaince and integration effort low, so I decided to program a solution on my own.
The solution uses the layout used by the webpage (www.discovery.jku.at), and it consists of a few php scripts, which use the existing mysql database (separate tables are used to store the data). Users can register using their name and email address for one or more courses. Registration details can be changed later using a link provided per email. The Number of participants to some courses can be limited; registration works on a first come first served basis.
Although I usually prefer to use existing solutions, this would have taken a lot of effort to integrate and maintain an existing system. In addition, existing solutions provide lots of functionality I did not need in this specific case (e.g. ticketing, payment, ...). Therefore, a small own solution (loc < 500) was the best way.