Ask HN: What tools do you use to alert users?

I'm curious what tools or services you use to alert end-users of something very important that's happening, for instance, a partial outage, a change in your T's & C's or policies, an upcoming deadline for user action, a scheduled maintenance, a temporarily disabled feature (e.g. experienced too much load), etc. Do you have a tool that enables this kind of thing without deploying new code every time it comes up?