I came across this product in a PC magazine.
PostureMinder uses any standard, off-the-shelf webcam to continually check your posture while you work. If you slump or lean consistently for a minute or two at a time, the software detects this and provides an an on-screen reminder to help you avoid spending long periods in these damaging postures.
I've not seen or tried it but it's had a good review in PC Advisor.
A useful add-on for your PACS workstation? It's only £29.
The website also has some good links to ergonomics sites.