RF simulation software can be categorized as
These are based off of open source circuit simulation software developed at University Of Berkeley in the 70s.
LTSpice from Linear Technologies is a free but closed source version of Spice. Linear Technologies distributes it free with models of their (excellent) switching power supply parts besides other models. LTSpice has a Yahoo user group where one can find answers to common questions. I tend to use LTSpice almost exclusively now.
B2Spice is a low cost Spice. I have used it.
PSpice got bought out. While student versions are cheap, the professional versions cost about $1000.
Spice simulations are only as good as the device models that go with it. In general Spice simulations are good only till 30-40 MHz or so. They also have the issue that the solution does not converge sometimes.
Am still writing this article - bear with me while I write descriptions of other RF simulation software.