stackoverflow (hdorio)

Fast answer:

sed ':a;N;$!ba;s/\n/ /g' file

1. :a create a label 'a'
2. N append the next line to the pattern space
3. $! if not the last line, ba branch (go to) label 'a'
4. s substitute, /\n/ regex for new line, / / by a space, /g global match (as many times as it can)