Whenever I deploy an Asterisk Server to a client, these five tools almost always accompany the installation.
3. Additional Sound Files
5. G729 Codec
If you have any other, must have addons, please let me know in the comments!
1. Dialplate (free or paid)
Having used the free version of Flash Operator Panel, this was an absolute breath of fresh air. This Windows desktop application is a very advanced soft-switchboard that enables your clients leverage the power of Asterisk through a very easy and intuitive interface. My recommendation would be to run this on a small form factor PC with a touch screen for the best experience.
2. CDR-STATS (free)
Transforming call records into useful information is what CDR-Stats does, and does well. The interface may be one of the most polished looking interfaces found on any open source project, and the support offered through the Google group is exceptional. Depending on your existing setup, getting it up and running can be a bit hairy, but with the excellent support and up to date documentation you should be up and running in no time.
3. Additional Sounds (free)
A lot of Asterisk users are not aware of the additional sounds pack provided by Asterisk. These actually come from Digium themselves, and are available for many different languages and accents. It helps to ensure that you can build a cohesive and comprehensive voice response system without the need to record any of your own audio files.
4. Zoiper (free or paid)
Everyone has their favorite softphone, mine happens to be Zoiper. In general, I am not a fan of softphones, but when I am forced to use one, this is my choice. The main reason for this is that it works extremely well with Asterisk, and even has the ability to connect using an IAX trunk, which makes it quite useful in terms of testing. It is also cross platform, including Windows, OSX, IOS as well as Android. The paid version includes support for G729.
5. G729 Codec (free*)
*may not be legal in your country depending on copyright laws
As far as I am concerned having G729 support on your pbx is not optional. Depending on where you live, you may be able to make use of the free G729 codec from http://asterisk.hosting.lv/. This free implementation of the codec has no limitations, and in my experience works just as well as the one from Digium.