Then there was Mail Goggles, which requires that Gmail users answer simple math problems before allowing them to send messages, in order to prevent e-mailing while intoxicated.
Gmail Labs also includes a Forgotten Attachment Detector, which will prevent messages that mention an attachment but don't actually include one from being sent.
And on Thursday, Google added Undo Send, a Gmail Labs experiment that prevents the transmission of an e-mail message for five seconds, to allow e-mail authors time to change their minds.
"Turn on Undo Send in Gmail Labs under Settings, and you'll see a new 'Undo' link on every sent mail confirmation," explains Google user experience designer Michael Leggett in a blog post. "Click 'Undo,' and we'll grab the message before it's sent and take you right back to compose."
Leggett warns that Undo Send won't recall a message that has already been sent, it simply holds messages for five seconds to allow time for second thoughts.
Google has even dug up some metrics to justify the addition: A company spokesperson pointed to a study showing that 87% of executives reported they have mistakenly sent or received an e-mail or other electronic message.
Google launched another Gmail Labs addition on Wednesday: the ability to preview YouTube videos, Picasa and Flickr links, and Yelp reviews from within Gmail messages.