I’ve used a couple of these myself, but some of the concepts haven’t even been on my radar. For example, it never crossed my mind to offload the actual act of sending an email, but there are some strong arguments for doing so. For starters, having another server send your email can be a lot more reliable (I’m looking into that myself for my Yii book Web site).
In the list, Thomas explains the role each tool serves. Even if you don’t have these specific needs (yet), give the post a quick read just to see what other people are doing and what should perhaps be on your radar for your existing or next project.