To get a copy, visit Linear Technologies web site. The program can be run under both windows and Linux. For Linux just make sure you have "Wine" installed, most modern Linux distributions include wine on their software repositories.
LTspice Group The LTspice group is now hosted on groups.io Originally this was hosted on yahoo groups and is a great place to go for help. In addition to the forums, the files section contains new models and downloadable circuits.
LTspice User ManualA comprehensive user manual (in pdf format).
LTWiki Contains many helpful topics relating to LTspice.
LTspice Getting Started Guide. A very informative getting started guide by Linear Technologies.