As someone who parses through a prodigious amount of e-mail every day, I'm constantly working with Microsoft Outlook to find e-mails and recipients, add new contacts, write myself reminders, and make sure I actually see those reminders even after I've left the office.
Three free or free-to-try add-ons are a constant help. There's Xobni, a search tool, Gwabbit, a contact management helper, and Google Calendar Sync beta, which helps keep calendar items current in Outlook, Google's Calendar, or both. Read more about the three killer Outlook add-ons here.
If Outlook is too conventional (or expensive) for your tastes, we recommend checking out Thunderbird, a free, five-star alternative from the people who brought you Firefox. There's also Postbox, a Thunderbird-powered variation in beta, which just received support for extensions.