Trojan-Downloader.Win32.BMPAgent.a appeared exactly a year ago today.
This Trojan downloader was interesting, as it came in a new package - it arrived as a BMP file. When the file was viewed, the malicious code contained within the image would be launched, which would in turn download and install other files to the victim machine.
Agent was written to exploit a vulnerability in the way in which certain versions of Internet Explorer/ Outlook Express process graphics files. Three months after the vulnerability was detected, Microsoft issued a patch for the vulnerability.
Although Agent had a novel approach, it never became really widespread, in great part due to the fact that Microsoft patched the vulnerability.