RPCEmu is a really important piece of software to me because it allows me to turn my laptop into a RISC OS machine when I am away from home. It provides an almost complete RISC OS machine under emulation for Linux, Mac and Windows. It can run the latest RISC OS 5.28 as well as 'classic' versions. There are several Easy-Start bundles with all you need to setup RPCEmu for RISC OS. The website states the software is alpha quality, but I have found it very stable in daily use.
So I am really excited that there is a new update (0.9.4). This adds some performance gains by improving the ARM recompiler and allows port forwarding. It also adds support for HFE format disk floppy disc images. There are also lots of bug fixes.
RPCEmu is available for Windows and Linux directly from the RPCEmu site. The Mac version is supported by another developer and you can get the latest from the github page (the main site has not been updated to show this).
There are still a couple of things it would be really nice to see in RPCEmu - top of my list is ShareFS and the ability to cut and paste text from my Mac to RISC OS and vice versa.
The RPCEmu website includes lots of developer information (and the source code) if you interested in experimenting with the code. There is also an option on the website to show your appreciation by buying the developers a Tea or Coffee.