Any company's Exchange server holds a wealth of information that could be used to build some very compelling tools, if only it was easily accessible. I've made it my mission to crack that silo and build some useful services, which has meant researching every way I can find of drilling in. Here's the approaches I've implemented or looked at:
Microsoft has reused the acronym for Messaging API in multiple different projects. This version refers to the interface for accessing mail data held on the same machine. In use since 1992(!), it's supported by all configurations of Exchange, but it's deprecated and requires an optional component in 2007.
It offers fast access to all tasks, contacts, emails and calendars for every user. The biggest obstacle is that it requires server software installation, and most Exchange admins are deathly afraid of touching a working configuration. There's also other same-computer access APIs like ExOleDB that work very similarly. Mailana implements a server-side MAPI downloader as one of its entry points.
EWS is the officially recommended replacement to local MAPI, but unfortunately it's only supported by Exchange 2007. Like MAPI, you can see all users' information as long as you have correctly setup the admin access account. It's a network interface, so there's no scary installation required. I've not implemented this yet.
The other protocol named MAPI, this one is the network interface that Outlook uses to communicate with the Exchange server. It's not officially supported as an API, but third-parties like OpenMAPI have reverse-engineered it. The great promise of this approach is that you could implement a proxy server that sits in between Exchange and its Outlook clients, picking out all the information you want as it flows across the network with almost no change to the existing infrastructure. The downside is that you only see information as its sent back and forth, which means no access to historical data. You can see all the types of information supported by Outlook. This isn't implemented in Mailana, but it's definitely very attractive and I hope to work on it soon.
IMAP is the standard internet protocal for sending mail between servers and clients. Many corporate networks have IMAP journaling enabled for Exchange to allow back-up services to pull down mail items for their archives. It gives you access all users' emails, but you can't see other items like contacts or meetings. This requires the admin to setup journaling on the server, but that's pretty low-impact. I've got IMAP support in Mailana, which as a bonus also lets me import Gmail accounts.
From a client-side plugin to Outlook, all of a user's information is readable through the Outlook Object Model. You only have access to information that's been kept on the user's local machine. The really nasty bit is that this approach requires installation on every client machine connected to the server, and you have to jump through some hoops on older Outlook versions to avoid annoying security warnings. I've got this implemented in Mailana.
Like MAPI/RPC, ActiveSync is designed to allow the transfer of tasks, emails and contacts between a device and the server. This means you only have access to a single user's information. It doesn't require any installation, and is enabled and supported on most servers. I've not implemented this.