For those who haven't yet launched into the vast and open world of EVE Online, PLEX is not only an in-game item, but also how players subscribe to the game. By either purchasing PLEX outright with real-world currency or earning it in-game, players are allowed to fly their ships for 30 more days. In other words, players aren't just donating in-game items, they're donating the item that allows them to play. To thank them for their contribution, CCP is rewarding donors with a special in-game shirt.
Those who want to donate can read more about the process over on the EVE community blog.