from the R contributed repository at this site which was used for packages not yet released to CRAN. All of the content in this repository is now available on CRAN, and it is only maintained as an archive for users of older versions of R pre-dating release on CRAN. It is also possible to install online within R, for instance:
rSpatial <- "http://r-spatial.sourceforge.net/R"
install.packages("spgrass6", repos=rSpatial)
Windows binaries will be made available using this channel, and older versions will be available as sourceforge file releases.