I have Reaktor sitting as an effect bus or insert on almost all my projects. There are some great reverbs, delays , and manglers. Check out the filters and delays from Dieter Zobel. Other recommendations are Boscomac http://boscomac.free.fr/
You do have to go through a lot of lightweight stuff on the user library. But it's worth trying. And the mastering effects in the standard library are good. With some users providing improved versions as well.