Emulators can be invaluble. Admittedly their main value is in allowing you to play old games again, but some of the better emus allow you to use serious applications designed for a different OS, vastly increasing the usefulness of your lump of hardware. Emulators that I personaly use on a reguar basis include:
Warajevo (DOS->Spectrum): A 100% perfect emulation of every Speccy operating system. Can use just about every file format circulating on the web and even has provision for connecting a tape player to your PC!
Redsquirrel (Windows->Risc OS 3.1): A very good emulation of a early ROS computer. Alows full use of just about any application and most simple games (more advanced games eg. Starfighter work, but slowly) as well as full access to the hard drive. A godsend for a ROS lover wanting to use Impressions/Schema on a PC or laptop. A buggy Risc OS 3.7 work-in-progress version is now available. Very impressive.
Archie (DOS->Risc OS 3.1): A ROS emulator for DOS. In my experience a bit harder to use than Redsquirrel and crashes more often (that may just be me). Unlike Redsquirrel (at the time of writing) can access the floppy drive and can print.