Microsoft now offers "free" Microsoft Outlook Web App clients for iPhone and iPad; the one catch though is you have to have an Office 365 subscription to use it.
Outlook Web App (OWA) is a client with that provides email, calendar and contact functionality. It works the same way that Outlook Web App in the browser does. However, according to a new Office 365 Technology blog post from Microsoft, the native versions provide "additional capabilities that are only possible through native integration of the app with mobile devices."
The new app is available for download at Apple's App StoreSource article