EHR, payroll, on-call management, paging, messaging, MDM, billing, mobile computing, HR, interfaces... pick ANY thing really, as almost everything will need to be hipaa compliant.
the healthcare system i work for has 650+ applications we support, I was hired as a developer yet we do very little development here. Hospitals like the idea of a vendor they can call and make fix anything, because its very costly to have SMEs for every single piece of software we use. many of them requires us to send our staff our for training yearly.
example: one vendor sells us diagnostic imaging software, but require us to buy a "certified" video card if we want their support... its just an old 90's nvidia card they rebrand and charge us $1500 each for.
Epic and Cerner are probably the largest EHRs. Epic revenues were 1.56B in 2013. While Cerner did 2.66B in 2013.
also, a lot of healthcare spending is government subsidized, which makes it even more lucrative.
Thx,
AC