SSH has a handy feature called agent forwarding that allows you to log in to a remote server and use the keys loaded into your local ssh-agent as if they were on the server. Unfortunately, this useful feature has a downside: it's not safe to use on servers you don't trust. Here are some alternatives.
While you're testing email functionality on a website, you don't want to accidentally send out mails to all your (or your clients') end users! There are a few ways to prevent this. Here's the simplest one I've found so far. It allows you to redirect all outgoing email from PHP to a local user (you, for example), so that you can read it with any ordinary mail client. You can also redirect to any arbitrary external email address you own.