Didn't know that Google Labs was shut down. Some of the Gmail Labs were useful - and some have probably gone mainstream by now. The one that reminds you if you use a word like "attached" in your email, but have no attachment, comes to mind. I tried at least half a dozen of them, may have only kept using one or two. Still, the Labs idea was good.

Undo send originated there as well.

I've been using it for years, so I did not realised they had closed Labs in the interim.

Right, good point. That has saved me a few times from accidentally sending an incomplete email - luckily not anything worse :)

Google Questions. I know it's been long ago and there were problems but when you needed it for real it worked wonders.