There have been quite a few cases of emails being sent in the name of Gmail (Google Email) users to their contacts which have not been authored by the holders of those Gmail accounts. Known websites that have beeing using this technique are desktopdating.net, yaari.com & gazzag.com. These websites send invites and other spam in someone else's name to their contacts. An easy way to spot such a false email is to see the details or properties - it will show the from address as firstname.lastname@example.org, but the mailed by server will not be gmail, for example it may be desktopdating.net.
These sites have also been mentioned in a blog post of 'Ill-mannered websites'.
If you can, block or filter these websites in your email accounts, web servers an any other online channels. If you have become a member on these, I would strongly recommend that you unregister and remove any personal or social contact details. Finally, if you do recieve any email mentioning links to these websites, do not click on any of these links, as that will start a program that attempts to read through your entire contact list and store it for spamming purposes. Simply delete such email, marking as spam may be tricky as that may block the email address.
Though this vulnerability has since been fixed by Gmail, in the time window that was open some spammers had been able to harvest and collect contact lists for many email holders.