Onyx by Titanium Software

I've been using it for years. It's safe to use as long as you understand what it's doing and why.

Every single thing Onyx does is done by running Unix commands in the background. These are all either known, or undocumented commands built into the OS.

I like to use it with every Mac we set up to turn annoying (to us) OS "features" off. Like animated windows. I neither need, or want to see folder windows flying open or closed. I just want them to snap on and off. Basically, I turn off as much eye candy as possible.

As far as cache goes, that is also safe when used properly. As mentioned by others, you don't want to do this at any ol' time. For starters, removing cache data is not a maintenance thing you do periodically in a "just because" manner. You only do so as a way to try and figure out why the OS or apps are misbehaving. Removing corrupt cache data can help with that. You are guessing though, since there isn't a direct way for the user to know whether most cache data is corrupt or not.

Doing it right is to close all running apps so the only items running are Onyx and the OS itself. When you click the button to remove cache files, it will ask you to restart the computer. Do it. Since you can never know when the OS may have been writing to the cache, immediately restarting removes the possibility of botching the cache by pulling out from under the OS. Once the Mac restarts, the OS will build a new cache file. But again, this isn't something you do regularly.