7-Zip. It can extract just about any kind of compressed file, including exe's and ISO's. It's also very helpful for things like HP print drivers where you don't need all of the additional bloatware. Just extract the contents of the driver download and when installing the printer in Windows, just point the printer install menu at the folder you've extracted the driver files to and magically you've got a printer installed without taking several hours.
Irfanview, which was already mentioned.
PuTTy, for telnet and SSH
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