The splancs software was originally written by Barry Rowlingson as an Splus package, as reported in:
ROWLINGSON, B.S. and DIGGLE, P.J. (1993). SPLANCS: Spatial point pattern analysis code in S-plus.Computers in Geosciences, 19, 627-55.
Subsequently, Roger Bivand translated the code to R, in which form it can now be downloaded freely as a contributed R package from the R project web-site.
The spatstat software is an R package written by Adrian Baddelely and Rolf Turner, which can also be downloaded from the R project web-site.
For an overview of these and other R packages, click on Task Views from the CRAN page and scroll to the "spatial" keyword.