Database of microRNAs (small non-coding RNAs), provided by the Sanger Institute.
http://www.mirbase.org/

TargetScan predicts biological targets of miRNAs by searching for the presence of conserved 8mer and 7mer sites that match the seed region of each miRNA (ref. 1). As an option, nonconserved sites are also predicted.
http://www.targetscan.org/