Digidesign Eleven Free is an amp modeling piece of software the comes free with Pro Tools 8. I've have the full version of Eleven as well, and while this is limited when compared to the real version, there is still a good amount that you can do with this one. Obviously since I have the full version I don't have a need for this, but I have looked and played around with it to see what the differences might be. It has the same sort of look and interface, but lacks all of the controls, presets and layers of the full version. You don't need to do anything to install the plug-in once you've installed the Pro Tools 8 software, as during this process it installs all of the free plug-ins that come with it. The interface of the plug-in is straight forward and easy to understand without needing to have used it for all too long. On top is where you choose your amplifier and cabinet types to model after, as well as set your main parameters of input and output level and threshold and release for a gate. It then has another slew of parameters to work with that include gain, bass, middle, treble, presence, and master, as well as speed and depth for effects that the different amps might have like tremolo. I don't think that a manual is necessary with the free version, as everything you need to know is right in front of your face and is pretty self explanatory.
Having just upgraded to Pro Tools LE 8 from 7.4, I'm now running Digidesign Eleven Free on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I run Pro Tools on a Digi 002R audio interface. However, since I have the full version of Eleven, and had it before I upgraded to Pro Tools 8, I don't really have a need for the plug-in and haven't run it beyond just checking it out, so I can't really say how taxing it will be on your machine. If you've got a solid system capable of running plug-ins well, I don't think you will a problem with it as long as you don't go overboard when using it.
The beauty of Pro Tools 8 is not only the new interface and features, but the free plug-ins that come with it like Digidesign's Eleven Free. While not as useful to me since I already had the full version, this is something that can come in handy for those who don't have any other modeling software. The sound quality is definitely comparable to the full version, it is really just the features that are limited. If you've got Pro Tools 8 I'd definitely recommend both this and the Sans Amp plug-in that now come free for any amp modeling that you might need to do.