Gmail – “People widget” has disappeared in Gmail

A sidebar widget would be nice, but short of that, there is a nice built-in feature for looking at past conversations with someone. When you're reading the email from them, hold the mouse over the contact's name and a small popup dialog will appear. In that, click "More" and a small menu will drop down. There's a button there that says "Recent Conversations." This should at least save you the time of having to go to the search bar and typing in their email address.

Plus sign is GMail's delimiter for aliases, which many people use for filtering and tracking spam.

For example, you want to make sure that your bank's emails are all legitimate so you'll register your email with the bank as johndoe+mybank@gmail.com and then create a filter that deletes all emails that don't match from:MyBank AND to:johndoe+mybank@gmail.com. Alternatively, when you register with a shady site you can provide your email as johndoe+shadysite@gmail.com and then create a filter to delete everything that is sent to that address.

However, beware that some sites have poorly-written validators on allowed addresses, which block emails with plus sign, which is actually an allowed character per RFC 5322. Google doesn't allow it simply because it's a reserved character in their system.