I don't think you need any additional software at all. If I recall right it's a simple edit to the php.ini file.
It certainly isn't software that costs you anything. I'm not the server guru though, so perhaps someone who is can chime in with how easy it is to grab imap
I stand corrected, you do need to download and compile one item. I put a link in the first post with a link to installing imap. Seems fairly painless though