Here's the stuff I use pretty much every day (granted this "kit" may be a little over the top for the average user just dealing with their own pc but most of it is free and all of it is useful),
Ubuntu live CD
Useful for testing "broken" hardware, data recoveries (it'll read/write every file system a consumer tech is likely to come across and doesn't care about file permissions), partition manipulation (using "Partition editior" - ie gparted)
Gparted live cd
Partition management (think FOSS PartitionMagic), also included in Ubuntu but useful if it's a slow machine or it wont boot Ubuntu for some reason.
Extremely comprehensive testing and repair disk. It contains everything from MemTest and hardware testing tools to hard drive regeneration and a live XP installation.
Bootable vista install - able to reset passwords on XP, Vista or W7.
Already mentioned, excellent for finding out what hardware you actually have to track down missing drivers. Also excellent for extracting license keys from the registry and stress testing hardware.
Anti-malware, perfect for getting rid of fake-AV software such as SecurityTool, just boot to safe mode and you're away.
Symantec Ghost / Acronis True image
Yes you have to pay for them but one or the other will usually get you out of some serious pickles when it comes to imaging windows installs from one HDD to another (faulty HDDs etc) and you can use it to keep decent image backups. On Apple Macs Carbon Copy Cloner is a good alternative.
Windows XP Home, XP Pro, Vista (one disk does all - just click past the dialogue asking for the COA and select the version you need), Windows 7 (RTP disk does the same as vista, if you only have commercial or oem disks you'll need the correct version for the license you're using. I.E. Home Premium 64 for a HP 64 key) also OSX snow leopard and Leopard. These disks can usually repair startup problems, kick off windows system restore, fix minor hdd issues and push shoves reinstall your OS.
Cleans out windows temp files / reg entries etc. Just remember to back up the reg before cleaning (it does ask).
AV removal tools
Norton, Kaspersky, AVG, MacAfee - they all make removal tools which will remove stubborn AV installs (old versions of Norton are bad for not uninstalling correctly and killing a machine's network)