WorldCoolers is actually a desktop application, which lets you receive notifications and participate in online rallies. If you like, you can also receive alerts online through an RSS feed, or with the help of a Firefox add-on.
From the website:

WorldCoolers is an online grassroots initiative whose goal is to raise public awareness to climate change. We are working to spread the word, spark fruitful discussions and bring this critical issue to the attention of policymakers and the media. We are looking to bring the work of existing on-going campaigns to online communities and the general public. WorldCoolers believes that by tapping in to the power of the Internet we can make a difference both on the personal level and on the global one.