Review on RBLTracker by Tom Lantz

I like how easy it was to get set up with my own account as well being able of customizing what types/domains you want alerts for (i have some specific sites i check regularly).

It's free but does take time getting setup properly so make sure your doing this correctly! There are not many features available yet or in development by rbltracker team however they do communicate via email fairly often which helps keep me updated about upcoming changes / feature additions etc - great customer support overall! Have been using them since 2016 when their first version came out & still continue today :).