• are free/open source,
• create checksums for multiple files,
• export checksums to a file,
• and verify md5 sums from a list of files.

In short: if you need a very good tool, then use the Swiss File Knife (sfk).

For creating checksums for all your files in the current directory and storing the individual checksums in checksum.md5, execute this:

sfk md5gento checksum.md5 .

And for validating the checksums of all files in the current directory, execute this:

sfk md5check checksum.md5

That’s it