I'm sure there's a bash oneliner for this, but I had no clue how to Google for it. ;)

leftmerge.py

""" ./leftmerge.py filename filename2 [delimeter]

Example input:

    filename contains

        talo
        sana
        kirja

    filename2 contains

        +N+Sg+Nom
        +N+Pl+Nom
        +N+Sg+Par

Output:

    A+X
    A+Y
    A+Z
    B+X
    B+Y
    B+Z
    C+X
    C+Y
    C+Z

"""

import sys

def main():
    left, right = sys.argv[1], sys.argv[2]

    try:
        d = sys.argv[3]
    except:
        d = ''

    with open(right, 'r') as F:
        lines = [a.strip() for a in F.readlines()]

    with open(left, 'r') as F:
        for l in F:
            for r in lines:
                print >> sys.stdout, l.strip() + d + r

if __name__ == "__main__":
    sys.exit(main())