I was considering a handful of nice keyboard trays on ebay, all with articulating arms, until I won a nice one yesterday for $5. They are a great investment if you want to prevent RSI, or just as a desk usability improvement. While they are very expensive to buy new, on ebay a few can be had for relatively cheap.
Here are the auctions I had bookmarked during my search. They are all considerably under priced IMHO.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=360155229406This one looks really solid (wood w/rubber coating), and slides back & forth on a ball bearing track. It also has slide-out 8" mouse trays on each side of the board.
$31.50 + shipping!
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=120413739488Same as above but without the mouse mounts. The tray is 21" and an M is 19", so it would be a nice fit, and the colors match.
$35 + shipping
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=360154155963Humanscale brand--very expensive new and from what I understand, the best. Auction ends in 4 hours, if you set a smart snipe you could probably get a steal on this. $25 + cheap shipping right now
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=280346894203Same as above, but used.
$50 OBO + shipping
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=150230921254Weber Knapp Easy Riser Tray. This is basically what I bought. This has a 30" tray, so you wouldn't need a separate mouse mount. This guy offered me exact shipping, so be sure to ask for that as well.
$40 OBO + shipping
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=350192941698Same as above, but without the mouse mount.
$30 + cheap shipping!
I hope someone finds these useful, they really seem like steals if you're in the market.