QuickMailer is an interesting tool for Mail.app users. It allows you to quickly send e-mail messages from the menubar, without having to open the Mail.app window. This saves you a lot of time. QuickMailer puts an icon on the menubar. When you click this icon, a window with three text fields will pop-up. In the first field, you can type in the recipient's e-mail address, then, in the second one, the subject, and in the third, the message. The app also lets you call up this window with a hotkey, which saves you more time.
I wrote a couple of e-mails to test this app and I was surprised at how well it worked. The app auto-completed e-mail addresses as I was typing them and it even has support for signatures. This can be good or bad, because Gmail also lets you add a standard signature to all your e-mails, and you don't want to have a double signature. So you may want to not use this feature on QuickMailer.
QuickMailer is best used for short e-mails that are simple and don't require you to add a CC or BCC e-mail address. Also, you can't choose what e-mail account you want to use to send e-mails.
I like that the menubar icon becomes red when you have unread messages.