Introduced in Spring’13 you can now reply to the chatter notification email to like the post. Just click reply and type “like” in the mail body as shown below. Do note that reply should only contain the word “like” (case insensitive). If it contains any other character other than “like”, it will be processed as a ‘comment’ rather than a ‘like’ on the post

Reply Chatter Notification Email to like the post