Recently some Wordpress plug-ins were discovered to have malicious, Malware, trojan code in them. Below is the list of of known malicious Wordpress plug-ins.
For the hardcore, geeky deets of what these malicious plug-ins do, check out Adam Harley's blog post. (Love the last name Adam! )
If you're aware of any others or new ones, please add a comment with the plug-in name(s), & version(s).