Snooze is an application that lets you set quick reminders with e-mail integration to remind you to reply to e-mails or follow-up. You can get reminded of replying or following up for a e-mail in two clicks. Also, you can set quick reminders which can be snoozed instantly.
Mia for Gmail (was Notifier Pro for Gmail) is a desktop Gmail client which sits politely in your menu bar.
