What do you use for uptime/performance monitoring?

There are quite some interesting SaaS tools on the market.

Very specific ones, which only test uptime and maybe SSL. More broader ones which also provide status pages to which users can subscribe and get alerts when something happens.

What do you use and works best for you?

Do you use the full feature set or mostly one specific feature (e.g. uptime and alerting)?

Or are you even missing a feature?

Thanks in advance for your time and answers! :)