Very customizable tool to keep your coding working. Fits well for teams of any size. By running your source code automated tests, you and your team can easily check if that modification will work into production environment as expected/designed.
Jenkins works with a vast range of languages such Java, PHP, Ruby, Python, Clojure, etc. There are tons of templates for each languages ready to be deployed into your own Jenkins' copy, avoid you to redo/search lots of its configurations.
Jenkins can runs Unit Testing, Functional/Acceptance/End-to-end tests; Frontend tests (Casper.js, Mocha, even Selenium is well supported).
There are tons of analytics plugins to keep your code quality based on language's metrics and best practices.
It's a free and open source software, so, you do not fear to get your code check by Jenkins.
Another import thing to mention is: Jenkins has at least 5 years of constant development by open source community. You can easily found Q&A on how to setup, configure, customize it on the Internet.
I do recommend if you have some deploy experience.
There is no SaaS version. You need to download and push to your own server to use it.
Also, it's a little bit ugly in a UI wise but it does not compromise in any way your daily basis use.