When called in to fix a computer problem at someone else's location, what should you bring with you to increase the chances of success?? Certainly, it helps to know something about the problem before walking in the door, but having a generic set of tools that will assist in a wide variety of situations is critical. Here is a list of what I put in my toolbox.

Hardware:

• ?Laptop - You need your own computer since there may not be a working one present. Dual boot Linux/Windows, although you can stay in Linux, generally speaking. Better yet, run Linux and have a few virtual machines with different OSes ready to go.
• Wireless (cellular) modem card - This is not necessary, but I like to have a high-speed, dedicated internet connection that keeps me from having to rely on the customer environment, especially if the network is the problem.
• Wireless (802.11x) card - For working with/sniffing wireless networks
• Hub - You can use a switch that has a sniffer port that allows you to see all the traffic, but a 4-port hub makes it easy to insert yourself into the network quickly and unobtrusively
• Ethernet cables - Probably at least two standard and one crossover, just in case.
• Computer screwdrivers - in case you need to open the case and look for evidence of physical problems, or pop out flaky hardware.
• Thumb drive with all the software described below for as many OSes as they are supported on - Since a 2GB thumbdrive costs so little, this is easy, and allows you to quickly copy analysis and dev tools onto other machines.

Software

• Standard network and comunication utilities - ping, traceroute, ssh, etc - helpful for checking the status of machines and for answering questions about networks
• Standard network service daemons - dhcpd, named, etc - helpful for allowing your laptop to pose as various services.
• Advanced network utilities - nmap, wireshark - for really looking at what is coming over the network, and analyzing hosts
• Standard? gcc toolchain - don't leave home without it
• A serious IDE such as Eclipse - I like Eclipse because I can use it to quickly examine Java, Ruby, C++, PHP, etc with all the plugins I've installed over the years.? If you have a Windows partition/VM and some extra cash, Visual Studio can help too.
• Linux rescue CD from your favorite flavor - That way you can boot into an OS where you can do whatever you want, including inspect partition tables, mount various drives to access content without needing passwords, and generally, take the machine's possibly broken configuration out of the equation to separate out hardware and software issues.
• Password crackers and recovery tools - This one can be ethically questionable, but when you need some files that some developer left in their account and they've left the country for a 1 month vacation, a customer will be begging you to break them out.? I recommend something to recover BIOS passwords, a zip password file cracker, and if you want to lug a big drive around, a generic password cracker that uses rainbow tables to break systems protected by weak hash-based encryption.

That's a good starting list.? I may post again if I add other tools that may be of interest, and feel free to add your own suggestions in the comments.