More information, and a basic rundown on what ACAP is and how to use it, is available here.
This is an essentially fully conformant ACAP server. It's capable of handling the entire ACAP specification. It does, however, store all the data in memory. This turns out to work surprisingly well, and usefully fast, but it probably wouldn't scale to enterprise level. It's had virtually no testing apart from my own (relatively heavy) use of it, so I wouldn't recommend using it in the enterprise just yet.
You can download it here, and read the documentation here, and the source code is all here, a subversion repository. The current version is 0.2.1. License is GPLv2+.
Is it a weirdly featured IMAP client? Is it a wannabe groupware client?
Polymer is an online, ACAP-enabled IMAP client. Configure it once, then run it anywhere. It'll run nicely over GPRS, DSL, or ethernet, and it runs quickly even in cacheless kiosk mode.
If you use IMAP to read your mail, you want to be using Polymer. Especially if you've multiple places to read it from.
You'll probably want to read this.
Other ACAP servers are few and far between. Of the three listed at Alexey Melnikov's ACAP server implementations page, the Cyrus one is currently unmaintained according to its homepage, and Eudora WorldMail's server is apparently "not true ACAP" - it's read only, for a start. Although Stalker boast of Communigate Pro as being highly standards compliant, their ACAP server is far from it. I suspect this is testement to the complexity of the server implementation - the RFC says that server complexity was preferred over client complexity wherever possible, and I can certainly see that.
This is not to say it's more complex than, say SMTP - SMTP is actually probably more complex in actual implementation - but it takes considerably longer to get an ACAP server programmed to the point where a client can access it, and the lack of clients in the wild makes this even harder. This is, of course, also why there are so many poorly written SMTP implementations.
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