PSD is Adobe Photoshop's native format. It preserves layers, masks, adjustment layers, smart objects, and the full editing state of an image. Opening a PSD with all these features intact outside of Photoshop is difficult — but viewing and basic editing is entirely possible with free tools.
For viewing only, Photopea (browser-based, free) is the closest thing to Photoshop that exists outside Adobe. It opens PSD files with layers intact, supports adjustment layers, blending modes, and even smart objects. It runs entirely in your browser and handles complex PSDs that other tools choke on. Bookmarking photopea.com is the fastest answer to "how do I open this PSD."
For basic editing, GIMP (free, cross-platform) opens PSD files and preserves most layers. It won't handle every Photoshop feature (smart objects, some blending modes, adjustment layers become rasterised), but for viewing, cropping, resizing, and exporting to other formats it works well.
Krita (free, cross-platform) is another option, particularly good for PSDs that contain painted or illustrated content. It handles layer groups, blending modes, and high-bit-depth images.
For quick previews without opening any application: macOS Preview and Quick Look display PSD files natively (flattened view only). On Windows, install the Adobe PSD Codec or use IrfanView (free) for thumbnail and preview support.