OpenSSH is the de facto standard implementation of the SSH protocol.
If you use ssh-keygen with the default options, it will generate a private and a public key that will work with virtually any server.
Unfortunately, out there are Windows users that bother Linux admins with .ppk key generated by PuTTY; both keys are stored in this single proprietary file.
You can convert this file and obtain standard OpenSSH key pairs using puttygen provided with the putty package available on many distros.
puttygen your_key.ppk -O [output-type] -o [output-file]
output-type refers to they key type, private-openssh or public-openssh.