I've been wanting SSL for one portion of my website, which is one single domain name on my VPS.
As most certificates look fairly expensive, I had looked at my domain name provider and had found a fairly good deal: http://www.dynadot.com/ssl/about.html
I would be looking at the AlphaSSL one of which provides an intermediate (chained) certificate for me to install I assume which is what I wish for.
I however have started to host a few small project websites for users and had started to wonder if I could possibly use this certificate for them as well, as it is on the same physical host and IP address, just differing by top level domain name that they give me.
Would I be able to install the certificate for all of them with this AlphaSSL certificate?
Also, would I be able to use this freely for imaps (especially as the domain names are not part of the picture) and secure all my IMAP users?
No, the SSL certificate is tied to the domain, not the host or IP address. You'll need to get separate certificates for each domain you want to host. If your clients aren't fussed about the domain that displays for their secure content, you can host all their secure content under a single domain of yours, like
https://secure.example.com/<customer>
, but that looks a bit naff these days.