IndieAuth.com lets you login by signing a random text with the PGP key linked on your web site. To use this feature, you have to publish the key on your page first.
Export your public key
Your PGP key in your keyring has a private and a public part, and you need to export the public one into a text file:
$ gpg --armor --output cweiske.pubkey.asc --export cweiske@cweiske.de
With that command, a file cweiske.pubkey.asc is generated and filled with the public key of cweiske@cweiske.de in ASCII format.
Since there is no real file extension for a public PGP key, I chose asc (extension for an "ascii armored file") which at least identifies the content encoding.
Link the key
On your website, add either an invisible link (<link> in the <head>):
<link rel="pgpkey" href="cweiske.pubkey.asc"/>
or a visible (<a>) one:
<a rel="pgpkey" href="cweiske.pubkey.asc">my PGP key</a>