Firefox/Chrome/Safari: EmailOracle isn't anything entirely new, but it is unique in offering a mail tracking service for individual users, rather than wide-focus email marketers. It monitors emails and reminds you when your recipients haven't replied to, or even seen, your message.
After installing EmailOracle's plug-in to your browser and authorising it through your Google/Gmail/Google Apps account, you'll get some new buttons and options inside your Gmail view. You can choose to track the reply to an email and get notified within a set number of days if it hasn't returned. For an extra bit of slightly intrusive yes-you-did data, you can insert an image into your email that will ping you and EmailOracle back, when the message has been opened.
EmailOracle provides 20 "tracks" per month to its free users, and its paid plans offer more, including more subtle email tracking images, like a personal signature or a 1x1 pixel. It's admirable in how it doesn't bludgeon your Gmail setup, like some other plug-ins we've seen, and could be useful for that handful of emails you send out each month that really need to be seen.