Being one of the oldest tools of its kind in the market, Nagios is extremely popular. This is usually a good thing because there are a lot of resources on the internet about it such as guides and configuration recipes.
jKool uses JKQL (english like query language) to search real-time and historical events, metrics and transaction streams. It includes verbs like GET, SUBSCRIBE, COMPARE, etc. Compare allows comparison of events, activities and metrics.