Acorn System 1 (http://speleotrove.com/acorn/acornEmulator.html ) - Like the Sinclair MK14 emulator, this one very cleverly emulates the System 1 but it still makes it very difficult to use.
Atom (http://www.stairwaytohell.com/atom/wouterras/ ) - Another freeware one and works perfectly.
BBC B There are a number of BBC emulators. Here are just a few:
Model-B (http://modelb.bbcmicro.com/ ) - Freeware. Emulates the BBC-B and B+. You can remap the keyboard so it is closer to the one in front of you but there is a limitation (at least in my version) that the shifted versions appear on the same keys so some options are prohibited.
Horizon (try via http://www.nvg.ntnu.no/bbc/emulators.php3#Horizon where there are lots of others) - emulates a standard BBC Model B.
Beebem (try the same link as above) - This is definitely the best in my opinion. It emulates every BBC from the BBC B to the Master 128. It has, so far, run everything I have tried on it. The keyboard mapping is near perfect so it is possibly the easiest emulator I have used.
Electron ElectrEm (http://electrem.emuunlim.com ) - A very good little freeware emulator.