You should consider this to be a neat toy, not something which you would expect to actually work. There are a few bugs still in the CPU code, and a group of instructions are not emulated yet. This means that a lot of games will just crash. You can play simpler stuff like the "Horace" games just fine, though.
For a far better Speccy emulator (PC only), check out my other project.
If you actually want the emulator, you can download it here. There is a load of documentation in HTML format, which you can either view here or download as a ZIP. Note that this is a Java application, not an applet, so you need the JDK (or equivalent) installed - you can't just use a browser (under UNIX, you might be able to use Netscape with the -java option, but I've not tested it). The best performance can be obtained by using a JIT compiler - I can get close to full speed using Symantec's JIT on my P133.
If you want to talk about the emulator then you can mail me at firstname.lastname@example.org.
Since I've had a few requests for the source, you can now download it here. Please bear in mind that this was my first attempt at an emulator, and it never really got finished. Just don't expect the source to be all that great, OK? This source code is released as public domain software. Do whatever you want with it (although if you do use it for anything, a mail would be nice).