Turn all commans into newlines:

cat <FILE> | sed 's/,/\n/g'

Remove all blank lines from a file (I alias this as removeblanks):

cat <FILE> | sed '/^$/d'

Trim multiple adjacent whitespaces and whitespace at beginning and end of line (I alias this as my reducespace command):

cat <FILE> | sed 's/^\s*//' | sed 's/\s\s*/ /g' | sed 's/\s*$//'

Turn all newlines into spaces:

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

This will read the whole file in a loop, then replaces the newline(s) with a space. It creates a register via :a and appends the current and next line to the register via N. If it is before the last line, it branches to the created register $!ba ($! means not to do it on the last line as there should be one final newline). Finally, the substitution replaces every newline in the a register (now the whole file) with a space.