The SNIP indicator is created by professor Henk Moed at CWTS (Centre for Science and Technology Studies), University of Leiden. Moed's article about SNIP is available at http://arxiv.org/abs/0911.2632. SNIP measures a journal's contextual citation impact, taking into account characteristics of its subject field, especially the frequency at which authors cite other papers in their reference lists, the rapidity of maturing of citation impact, and the extent to which the database used for the assessment covers the field's literature. The current version is a start version of SNIP. CWTS continously developes the indicator to deal with its weaknesses.
Scopus Journal Analyzer publishes SNIP values for journals from 1999 and later. SNIP values for journals included in Scopus are freely available at http://www.journalindicators.com/Default.aspx.
In 2012, SNIP was revised, leading to some changes in the way it is calculated. These changes are explained in another paper (an open access preprint is available here).