Sure, and a bunch of those are already on the web, though it depends what you're trying to do e.g. mail could just be a view of an existing service (in which case it's just making the existing interface mobile friendly), or it could be an actual client (using IMAP/SMTP), in which case it should be native, rather than having some weird intermediary (which if you want to to do IMAP/SMTP via the web, you need to do). Personally, I'd rather native apps with well defined interfaces that use well defined standards (e.g. IMAP/SMTP/ical), rather than some hacked together web version. But people like reinventing wheels...