I built this over the weekend. You can use it to convert any amount to or from BTC, or just to see the current exchange rate.
Try putting in the number of Bitcoins you have and seeing the dollar value change (hopefully rise ) over time. You can also convert to Bitcoin by entering a USD amount (more currencies coming).
Check it out at preev.com.
Let me know what you think! Comments, questions, and criticism welcome.
P.S. Internet Explorer support is pretty minimal (for now).