Here are some free DICOM and medical imaging software packages that I have used and like.
* Clear Canvas - Clear Canvas is a very nice web based PACS and viewer. The free edition comes with all of the standard image manipulation tools such as window and level, zoom, pan, and measurements. It even supports multi-frame images such as echocardiograms!
* DICOM Works - DICOM Works is a free image Window's based image viewer. DICOM Works will allow you to convert a DICOM image into a JPEG. It will also let you edit DICOM tags. DICOM Works has a nice standalone feature that will allow you to create image CDs with a DICOMDIR file and an integrated image viewer. Very handy. DICOM Works is very good about opening corrupt files that other programs crash on. If DICOM Works can't open it, nothing will.
* Irfanview - Irfanview will allow you to batch convert DICOM images into jpegs. (and other formats)
* DCMTK - The DICOM Toolkit is more orientated towards programmers; the full source is included. It does include binary tools that will allow you to do DICOM pings, receive images, send images, and do all sorts of transfer syntax and format conversions.