Software download: http://www.linear.com/designtools/software/#LTspice
A short tutorial is the LTSpice Parameter Sweep Tutorial by Andrew Herdrich, Department of Electrical and Computer Engineering, Portland State University, January 7, 2007, Version 2. The examples below are taken from that tutorial.
Parameters and parameter sweep are defined by directives (commands starting with at dot).
Constant parameter:
.param C=1uF
Common loop expression, e.g. to test capacitors from 1μF to 10μF in 2μF steps:
.step param C 1uF 10uF 2uF
Parameters from a list:
.step param C list 0.47uF 1uF 2.2uF 10uF
Logarithmic (common log, base 10) parameter variation, e.g. 8 steps from 100nF to 1uF:
.step dec param C 100nF 1uF 8
logarithmic (base 10) AC sweep from 1 Hz to 100kHz with 10 steps per decade:
.ac dec 10 1 100k