SQL Server DBI interface for R, based on Microsoft's JDBC driver.
These packages are required: DBI, RJDBC
You will also need a working Java runtime, either 32 or 64 bit to match your R runtime.
Here is some example code:
con <- dbConnect(MSSQLServer(), 'myDBServer', user='sa', password='mypw')
dbListTables(con)
dbGetQuery(con, "select * from myDBName..my_table")
dbDisconnect(con)