If that's your most important message for today, you can also drag and drop it at the top of your inbox, so it's front and center. You can also add subtasks to it, such as "review property listings to view tomorrow" so that you don't have to reread the whole message when you're ready to reply. ActiveInbox puts new buttons in Gmail that let you set a deadline of "today" on that message. You don't want to deal with that message right now, but it's important to do today. Say your realtor emails you asking what properties you want to see tomorrow. Is your inbox really a to-do list? ActiveInbox is an add-on for Gmail that gives you options to turn mail into tasks.
All the while, the interface you use to access your email doesn't change significantly and you don't have to download a new app to use it. The more you use SaneBox, the smarter it gets. When it sees a new message that may be unsolicited or of low importance, SaneBox sends it to a different folder, so the email never shows up in your inbox. When you receive emails that are important, they go into your inbox. SaneBox is a good example this service goes into your email on the back end and categorizes messages based on their importance or other criteria you set. In these cases, you use your email account as normal but add something onto it.
Other items in this list are better described as tools, services, or plug-ins.