Publications for TRANSFAC
2008 Brief Bioinform
2006 Nucleic Acids Res
2003 Nucleic Acids Res
Department of Bioinformatics, Medical School, University of Göttingen, Goldschmidtstr Göttingen, Germany
Probably one of the best annotated and curated resource of transcription factor binding preferences (sites and motifs). The free version (v7.0) is very limited and outdated (year 2005). If you want to have online access to the last version, the academic price is more than 1000$. There are other open access databases that can be considered before paying that price.