I said I won't be posting "fixes", but this one is way too annoying not to help you fix it.
If you're using Pidgin with MSN, you probably got lots of "omega.contacts.msn.com" certificate errors today. Here is how to fix it.
1. Download this omega.contacts.msn.com certificate.
2. In Pidgin, go to Tools > Certificates and remove the existing "omega.contacts.msn.com" certificate. Then select "Add" and add the certificate downloaded under step 1 and when prompted to enter the hostname, enter: "omega.contacts.msn.com":
Now click OK and that's it!
Update: Pidgin 2.7.6 has been released, bringing a fix for this issue. See: Install Pidgin 2.7.6 in Ubuntu (With MSN Certificate Fix And Multiple MSN Logins)
Thanks to Lazonakf for the fix!