It would help to have a little more information about your iMac...which model do you have, how much memory? You can get the model from the apple in the upper left corner of the menu bar, About This Mac, More Info, System Report and then what does it say form iMac...the numbers following iMac please.
If you have an older model with less than 4 GB ram, that could be a reason for slow behavior with Mountain Lion...it works best with at least 4 GB ram.
Model Name: iMac
Model Identifier: iMac9,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.66 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache: 6 MB
Memory: 4 GB
Bus Speed: 1.07 GHz
Boot ROM Version: IM91.008D.B08
SMC Version (system): 1.37f3
Looks like you have enough memory, but you can check further on what that is doing by going to Launchpad, Utilities, Activity Monitor while you have your usual software open and see what the memory usage is and then what is the load on the CPU. If you see very heavy CPU load there isn't much you can do other than restrict the software usage to single programs at one time...if there are a large number of page swaps in and out going on with the memory then you may need to look at more ram.
You can also think about what processes do you have running simultaneously...mail pushes and checks, things like that using resources.
Byt he way, did this slow behavior start after installing Mountain Lion?
I use Memory Scope which is a Free App from the App Store & Also Bought iBoostUp, there's also CCleaner which is very useful since it cleans out your browser cache which can also clutter up & cause your mac to slow in Performance.
Apple said me that memory cleaners (expecially mackeeper) damage osx system, so do not download it!! if you have the apple care still active, go to apple store for a check up... they will certainly help you
ad96 is correct.
PCs are not Macs! So called memory "cleaners" and hard drive " cleaners" and things like MacKeeper are not needed on Mac OS X and are considered "crapware".
With the exception of OnyX for OS X, any type of "cleaning" apps are unnecessary and can actually cause potential harm to a perfectly running OS X operating system.
There are other ways to deal with memory issues and thinning out hard drive data and other system performance issues.
Ok..Let me be clear...CCLeaner is avaible for Macs as well as for PC's..Since i've owned BOTH versions i've never had a Problem with the App it's not spyware or Malware, I also Use Dasiy Disk, & Yes I HAVE Onxy also plus I use Memory Scope. I've owned my iMac for OVER 2 Years and have NEVER EVER....had ANY Software wear issues WHAT so ever and my Mac Runs Smoothly. Only Additon that i've added was A Memory upgrade which only cost $50 for upgrading my iMac from a 4GB to a 12GB. if memory cleaners aren't needed then how do you clean our your Net cache,Net History or cookies & History w/o having to go into a bunch of useless gestures when CCLeaner is just as Simple an App to use. I wouldn't mention it if i KNEW it was spyware or just bloated Utility Apps. It works just find on my Mac & will work fine on yours; you just need to see which is the best option for You.
All of this is for saying that in osx is not necessary to use any memory cleaner. If your Mac is slow is not for that. I'm sure. Mac is not windows! Windows must be cleaned usually otherwise Mac is autocleaned. If it is slow there must be a cause.
I've never had a problem with my Mac Hardware or Software issues what so ever, but to each their own, my Mac works Fine so...what works for works for me. Autocleaning only pertains to the Safari Browser which i don't use anymore because sometimes it slows down my browser experience, hence why i use Modzilla which IMP runs better then Safari. But again what works for me...might not work for others.
Mozilla has it's own delete caches, cookies and histories functions in the browser app, also.
I use and prefer Mozilla Camino over Safari, myself, but have Firefox, also.
Been using an iMac for two years!
Well, a lot of us here have been using Macs for a lot longer than that!
Do what you will.