ezGroups is designed to make downloading files from newsgroups as easy as possible. It will automatically download and decode yEnc pictures. If you register the program, it will also download video and audio.
NewzToolz is a free utility program that can decode yEnc, RAR, and PAR files that you download with other programs such as Outlook Express. It is also a good replacement for Outlook Express, so that you can click through picture newsgroups without having to go through extra steps to see yEnc pictures.