Successive repeated lines can also be eliminated at the (unixy) command line with uniq infile > outfile, so long as you don't run the data through sort first.

Also note that, of the solutions provided thus far, the hash-based option is the only one which will both eliminate all duplicates (printing only the first appearance of each line) and also preserve the original order of the (remaining) lines, which may or may not be significant to you.