Guestbook - will create a comment like "Hello, nice website." + leave a backlink
Referrer - will fake the http header with a referrer entry pretending a link is set on your page. In that was it will show a backlink on there page as well.
URL Shortener - will take your URL and make a shorter redirect URL from it. You can use this URL instead of the real one. Usually they redirect with 301 and will carry the PR and stuff with them to your new URL.
Comments
Guestbook - will create a comment like "Hello, nice website." + leave a backlink
Referrer - will fake the http header with a referrer entry pretending a link is set on your page. In that was it will show a backlink on there page as well.
URL Shortener - will take your URL and make a shorter redirect URL from it. You can use this URL instead of the real one. Usually they redirect with 301 and will carry the PR and stuff with them to your new URL.
Whats useful for you is up to you to decide.