Infinite Mac is a collection of classic Macintosh and NeXT system releases and software, all easily accessible from the comfort of a (modern) web browser.
Pick any version of System Software/Mac OS or NeXTStep/OPENSTEP from the 1980s or 1990s and run it (and major software of that era) within a virtual machine. You can also run a custom version with your choice of machine and virtual disks. Files can be imported and exported using drag and drop, and System 7 and onward have more advanced integrations as well – refer to the welcome screen in each machine for more details.