Home server/nas box,
Sell for funds towards the next build
Spare hamster cage? (I'm running low on idea's now)
I tend to sell my/our old ones on to fund my Modding/fan habit,
although watercooling seems to be my next hit
**Edit** Dual boxing on mmorpg's
Extreme couponing! ;-). Apparently those people on that show have multiple cheap computers for that reason. Other than that, can't think of anything really. Experimental machine (practice overclocking, etc.), download dedicated machine if you have router issues, for somebody else in the house (we have 2 desktops and 4 laptops with 5 family members).