There are always situations where you'd want to route the audio from another software, such as YouTube, into Ableton Live. This tutorial explains how you can achieve this on a Mac.
For those of you working on a Mac OS X with a big plug-in library there's now Auganizer. This app allows you to organise all your Audio Unit plug-ins however you want. This way you can come up with your own way of sorting them to find the ones you want to use in a production much faster.
One of my favourite plug-ins when I was still producing on a Windows machine was dblue Glitch. Now the plug-in has been revamped and is also available for Mac OS X and even Linux. In 32- and 64-bit. It's no longer free, but Glitch 2 comes packed with new features.
A week ago I wrote about Eric Ameres's Max for Live devices that allow detaching the Session View or its mixer and even individual devices. He's been busy developing a standalone version for OS X that you can use with all versions of Ableton Live without the need for Max for Live.
If you simply want to use the built-in sound card, the process is pretty straight forward and you can skip the first two steps. If you have an audio interface, read on as there's a little more to take care of.