One of the most productive tools I have discovered (and then never stopped using) is Boomerang for Gmail. If you are like me and send and respond to lots of emails per day and use Gmail or Google apps (if not, you should migrate to gmail asap), this is the right tool. Boomerang let’s you configure emails with one click so that if the email is not replied in the timeframe you desire (one day, 12 hours, etc) that email will come back to the inbox or will be resent automatically. Specially if you manage people or sales, it’s excellent for that followup. It also let’s you schedule emails for being sent later (perhaps it’s not so “professional” to send business proposals at 3 am), add tracking to emails and many more tools that will make it easier for you to track emails and tasks. It is free for gmail for up to 10 boomerangs per month, if not, the pro version is USD 5 for Gmail and USD 15 for google apps emails. It’s one of the best paid services I use and strongly recommend.
Try it and let me know what you think