Easy Wake On LAN Manager is a fast and free app to quickly send a wake-on-lan packet to any mac address on your network.
Simply enter the mac address of the device to be awoken (or select from a list of commonly used macs) and send a wake packet with one click.
You can download it from the Android Market here.
Update 4/2012 This is now open source and hosted on GitHub. Check it out at my GitHub page.