Google Hungary has a logo for the Hungarian Revolution today. The logo is a picture of the government offices with the Google logo in the Hungarian flag colors.
The logo links to a search query for [1848-49-es forradalom és szabadságharc].
The revolution took place between March 1848 and October 1849.
Forum discussion at Google Blogoscoped Forums.
I should add, Baidu, the large Chinese search engine, has a special logo for Great Wall of consumer rights, the livelihood of the tree alive. It links to a blog post.