Replace all newlines to space except the last

You can use paste -s -d ' ' file.txt:

$ cat file.txt
one line
another line
third line
fourth line

$ paste -s -d ' ' file.txt 
one line another line third line fourth line

You can use tr to replace all newlines to space and pass the output to sed and replace the last space back to a newline:

tr '\n' ' ' < afile.txt | sed '$s/ $/\n/'

Re-implementing vonbrand's idea in Perl, provided the file is small enough:

perl -p00e 's/\n(?!\Z)/ /g' your_file