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Twitter is deeply embedded within iOS 5, and available to install from the Settings. No longer is the App Store required to download and install -- it is ready to go from the word, well, "go". From almost anywhere within the operating system, you can bring up available options and tweet your pictures, notes, messages or nearly anything for that matter.
Email has been improved with previous HTML redding capability, but now HTML email creation. Crucially, you can indent text but add emphasis on words -- such as bold, italics or underline text -- simply by selecting it. Because iCloud is heavily integrated, you can set up an iCloud email account as well as the other email accounts, from Exchange to IMAP or HTTP web email.
Under the 'PC-free' ethos that iOS 5 seems to have, users can now delete, add and rename calendars directly from an iOS 5 device; previously requiring a iTunes connection to do this. Attachments are also accessible through the calendar without the need to switch applications, and of course -- iCloud integration, so all your calendars synchronise across devices.