Someone I know is leaving their current job, which reminded me to repost this piece I wrote a while ago (by amazing coincidence within days of leaving that job!). May it help you in carefully crafting that heartfelt Goodbye! letter that we all write from scratch every time and which always ends up sounding the same as every other one we've all read.
http://chetchat.blogspot.com/2010/05/corporate-survival-guide-retreatise.html