I personally use the Yumi multiboot software to collect a list of useful gadgets even if you don't plan to use a USB (which I would recommend since its a more widely accepted median than CDs) you can take a look at the list and scroll down near the bottom for more tech oriented tools people use.
There are some risks in recommending programs that I have to preface by saying the following.
-I'm assuming you are not using anything for malicious intent.
-I'm also assuming you will research these tools before bombing your own or worse someone else's HDD into oblivion.
As such you can always find a use for the following.
-A Repair disk for every OS release by windows especially the ones used by the main demographic of common users(don't bother with anything pre-xp). (install disks with the repair feature) Sometimes the installed system has issues entering its repair features and this I've found to be very useful to have on hand.
-Partitioning tool such as GParted. One of the most widely accepted tools for Hard Drive partitioning and formatting as far as power users go. Very versatile but very dangerous. IT WILL LET YOU FORMAT ANYTHING please be careful can be used to extend xp and vista passed the gb limits imposed by the os. Not sure how stable that is its not a common request.
-A few linux live cd distros, these I use mainly to remove or modify files without having to pull out the physical drive and connect it to another computer. Extremely useful when dealing with corrupt registries which windows has backed up but never uses... or temporarily removing SAM files which leads to my next point.
-User removal tools, probably the most common problem ever to exist. They changed their password now their SoL. I wont list these but these live disks/usbs can recover most user info from vista and older, and remove them from windows and up. There are a few things to keep in mind, always verify the person owns the computer or you just became an accomplice in his crime and courts wont think to favorably of you. Second, Vista was the turning point where microsoft hadn't yet started using salted hashes. What this means is there was only 1 encryption process used without any additional bits of unique data resulting the need for only 1 decryption table. When windows 7 released they added in salted hash's meaning the brute force approach was no longer feasible as each computer would need to be individually assessed to create a table. I'm not sure if they ever released that it was per computer or per password for unique salted hashes its been a while since I researched this. For Win 8 and 10 you can remove the password from the user but only if they are not using the Microsoft sign in feature and are using actual user authentication with the computer only.
Otherwise its in their court to recover their password.
-stand alone installs or offline installs for your favorite list of antivirus/malware/etc scans. While annoying to have to keep changing the definitions manually to allow for the latest detection definitions, some computers either have a slow connection (yes those dialup people still exist...) where downloading definitions would take you hours or can be infected to the point that you are unable to access the internet correctly or reliably. I've had virus's redirect just about every website I typed thwarting my efforts to obtain what I knew would fix the problem. Even outdated lists can sometimes get you stable enough to fix the issue saving you time.
-memory test software, little difficult to read but well worth the time if you've ever ran into a faulty ram stick before. Mem64 i think its called, seems to be a common choice.
-Hard drive checking software is great. One of the best non SSD drives on the market right now is a Seagate drive which is cheap and has decent space. Some companies like seagate have hard drive life tools that check the drives for degradation and in some cases repair and remove the problem entirely. Western Digital also has their own software. (these programs don't generally work with raids, if you want to skip to the end of any warrenty call with these companies tell them its a failed drive in a raid config they'll just skip to the end of the conversation rather than help troubleshoot the drive, dirty but nice trick)
-Last this website has an amazing list of programs that I recommend sifting through which can be found here: http://www.bleepingcomputer.com/download/windows/
Just my list/recommendations.
Would also suggest a toolkit for pc repair. Looks a thousand times better than asking if they've got a screwdriver laying around that's half covered in rust oil and other grim you really shouldn't put into your computer...