Development and support of TOSBOX ended in 2000. The old files can still be downloaded, but no more bugfixes or features are going to happen.
There are several Atari ST emulators available, and most are designed for game use. Tosbox services the productivity end of the Atari spectrum. It does not emulate all of the native ST hardware directly; however there are benefits to this approach. Access is allowed to all DOS filesystem devices including ZIP drives and arbitrarily large hard drive partitions. Several high-resolution video modes that were not native to the Atari hardware are implemented, along with the standard ST modes which are included for compatibility.
Before you can run Tosbox, you need to have an Atari TOS operating system image, which is typically a 192k or 256k file copied from the original Atari ROMs. The emulator package comes with a utility that helps you create such an image on a floppy if you already own an Atari ST. Images are also available widely (and I think illegally) around the 'net, but I'll leave it up to you to hunt those down if you want to do that.
Shareware fees are no longer accepted, and registration keys are no longer available.
Tosbox online documentation
Frequently Asked Questions
Download current version 1.10a
Personal homepage (at Iowa State University)