Doesnt exist for mac, ive searched alot, found about 10 application of which none were able to show the real time frequency.
Its not really necessary anyway. Tell us what CPU you have and I or someone else can tell you the turbo boost frequencies if only one core is active, two cores etc.
There was one app that could do it, Coolbook. I'm not sure if it's been updated for Lion or Mountain Lion. Another one might be CPU-X. It's a port of CPU-Z, but it hasn't been updated since 2007/2008. If you can find a copy of it, you could try it as well.
PS: CPU's in Mac OS X systems can and do run with 0% usage on at least one core. If there's only two active threads and four logical cores, two will be at zero percent. It's rare, but it does happen.