If you haven't heard about it before, IDA is a Windows, Linux or Mac OS X hosted multi-processor disassembler and debugger. It's widely used in RE community and by security researchers.
https://www.hex-rays.com/products/ida/support/download_freeware.shtml
Notably there is no plugin support.The freeware version of IDA v7.0 has the following limitations:
- no commercial use is allowed
- lacks all features introduced in IDA > v7.0
- lacks support for many processors, file formats, debugging etc...
- comes without technical support
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Previously we had v5 and only for Windows:
https://out7.hex-rays.com/files/idafree50.exe
Now we have v7 and it's available also for other platforms:
https://out7.hex-rays.com/files/idafree70_windows.exe
https://out7.hex-rays.com/files/idafree70_linux.run
https://out7.hex-rays.com/files/idafree70_mac.tgz
Nice move by Hex-Rays!