Stew Benedict's article serialized on CrossNodes over the past month has dealt with a roll-your-own approach to running IMAP services on *NIX servers, but of course, you can always look for an out-of-the-box solution. Fortunately, there are a number of well-regarded entries in this vendor space.
The most obvious disadvantage to purchasing software as an IMAP solution is money. The methodology as presented by Stew is free -- after you factor in your time commitment, of course. Additionally, while you will be able to find support in the form of gurus who lurk within Usenet news groups and similar resources, you won't have a specific vendor whose job it is to get you out of a jam should one occur.
Another way of handling IMAP under *NIX is with software that add functionality to already existing services. For example, Courier IMAP is a server that provides IMAP access to systems using Maildirs -- the directory-based mail storage format. Issued under the GNU General Public License, you can't beat the price.
However, by purchasing a ready-to-roll package, you can save yourself some time, and ease the configuration and interface issues; making it easier for less advanced administrators to implement and maintain. Selecting a hearty, feature-laden package can provide one-stop shopping for your communications needs.
One of the most popular of these is Stalker Software's CommuniGate Pro. CommuniGate Pro can run on any number of *NIX variations as well as Apple's OS X, Windows, and BeOS. More than an IMAP solution, the product also supports POP, LDAP, SMTP, HTTP, Domain Name Resolution, mailing lists, and more. Trial versions are available at Stalker's Web site.
Probably the favorite among the guru crowd is Sendmail, the original version of which which defined the way messages are sent through the Internet. The company has grown over the last 20 years now offers commercial packages such as the Single or Multi-Switch Consoles or the Sendmail Advanced Message Center targeted at enterprise and ISP-level networks.