This article covers some of the things troubleshooters would need during their everyday work. I’ve also listed links to some products and applications as an example. It is by no means an endorsement of the same.
OS X Install Disk: This is probably one of the most important tool that a troubleshooter must carry around with him/her. Ideally it would be one disk with multiple versions, each corresponding to a specific version of the OS. Exactly how many versions depends on the situation, for example one can have all instances of the present and previous 2 versions of the OS (For example OS X Mavericks 10.9, 10.9.1, 10.9.2, 10.9.3, 10.9.4, 10.9.5 OS X Yosemite 10.10, 10.10.1, 10.10.2, 10.10.3, 10.10.4, 10.10.5 OS X El Capitan 10.11, 10.11.1)
Thunderbolt & FireWire Cables: These come in handy when it comes to transferring data. These cables are necessary when you wish to perform target disk mode troubleshooting. Make sure you have the correct version of the cable.
Portable Storage Device: Along with the cables mentioned earlier having a portable storage device with a large capacity is useful. Mainly when it comes to taking a back up. The user may or may not have a storage device available for this. (Ideally, taking a backup of the data should be done by the user on his/her own storage device.)
Internet access via USB Dongle: Yet another important tool. Application & OS upgrades require internet access. This will also come in handy when you are trying to perform network troubleshooting. Make sure there is sufficient data available for upload/download. Optionally one can carry multiple Internet dongles from different vendors.
Portable Power Bank: This is more useful for portable devices. Carry one with a large capacity, enough for multiple recharges or charging multiple devices.
Display adapters: Necessary to troubleshoot when the main display isn’t working or to heck the display ports. Carry all combinations HDMI to VGA, MiniDisplay to VGA, MiniDisplay to HDMI, HDMI to HDMI and any others depending on the device to be connected to.
Secondary Display: This might not be practical as displays tend to be very large and not necessarily portable. However, one can carry a small pocket projector. A secondary display is always handy as it reduces dependency on the user/client to provide one.
Power adapters: Not strictly required but can come in handy. Useful for checking if the user’s power cord is functioning properly. Make sure you carry all the different versions of the power cords.
Lightning & 30 pin cables: Again not strictly necessary but would be useful for checking if the user’s power cord is functioning properly. Make sure you carry all the different versions of the power cords.
MacBook Pro &/or iPad: Carrying your own Mac & iPhone/iPad is very important. Load these devices with various tools required to diagnose and/or fix issues.
Disk Drill: http://data-recovery-software-review.toptenreviews.com/mac-recovery-software/mobile/disk-drill-pro-review.html
Airport Utility: https://support.apple.com/kb/DL1664
Airport Utility: AirPort Utility by Apple