EU nationals do not require a visa to enter the Federal Republic of Germany. You can find an overview on visa requirements depending on nationality in several languages at the Federal Foreign Office:
Should you need a letter of invitation in order to obtain the required documents to attend the conference, please contact us via ICANN2014 (at) informatik.uni-hamburg.de specifying the necessary details. We will be pleased to send letters of invitation to accepted authors. Please note that letters of invitation do not imply any financial support.