Monitor arms. I've got http://www.ergotron.com/Products/tabid/65/PRDID/355/language/sv-SE/Default.aspx
Also, a good chair, desk, etc. Vital if you spend as much time in front of the computer as many here probably do.
Monitor arms are awesome. After using them for a while I abandoned the desk entirely and just bolted them to the wall, allowing me to add a couch to my room instead. Warning, this advice probably not viable if you actually use your desk for stuff other than putting monitors on.
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: