A coaster that:

• has drainage, to collect your cup's condensation (without becoming soaked and gross, like coasters of some materials)
• is slanted inside, so both the collected condensation and the contents of the cup will collect near one edge
• has a drain hole above the lowermost point of the inside, so you can easily pour out collected condensation or other liquids

Works particularly well for chocolate milk and similar beverages—you can drink more of what you poured if you let it collect to one side, and this coaster helps you do that.