Daemon Tools Lite is still available for free. During installation, ensure you select the "Free License" (with ads) option. This allows you to mount ISO, VHD, and MDX files without needing a serial number.
While the allure of finding a "Daemon Tools Lite 501 serial number txt" is high, the reality is that most of these files are outdated or dangerous. For a stable and secure experience, stick to the official free version or use the native mounting tools built into modern operating systems.
If you want a powerful tool without any licensing headaches, WinCDEmu is an excellent open-source alternative. It is completely free, supports unlimited virtual drives, and doesn't require a serial key.