There are two apps that I highly recommend for iMac and MacBookPro users. I have only been a MAC user since June 2012, having replaced my Windows Desktop with a mid-2011 27" iMac.
The first app is called 'Temperature Gauge' by Tunabelly Software. It is available from the App Store for a small fee of £2.99GBP / $4.68USD. I started looking for such an app when my iMac was extremely hot to touch. Having been a Windows PC user, I was not used to this and was later told that imacs do get hot as they disperse heat via its aluminum casing. Still, it is a recommended app. It shows temperatures for CPU cores, GPU, optical drives, LCD monitor and Hard Drives. The app will also show a nice visual display of the temps with a marker showing the maximum it achieved. Fan revs are also visible with health indicator bars (green, yellow, red). You can have warnings posted when it reaches a certain temp and it will auto start in the background (no dock icon, just a menu bar icon) upon logon.
See post attachements for a picture of Temperature Gauge.
The second app is called 'SMC Fan Control' which is a popular fan speed modifier for apple systems. There is some debate in the Apple world about the need to alter fan speeds, but I personally have found this little gem to be a savour to keep the iMac under control in terms of temps, and prolong the life of its components. So why SMC? It only allows you to set minimum fan speeds. This means the system can still regulate the speed if it got too hot as that would be a max speed. Also, it will not let you set a min fan speed below the Apple defaults. It is by far the safest and most reliable fan modifier out there. You can change the readings between F and C degrees and even created speed profiles. This is handy as you could make a profile for browsing, video editing and gaming. Profiles (or favorites as they call them) can also be auto applied under various conditions. Again, it auto starts after login and remains in the background with a little status menu icon.
See post attachments for a picture of SMC Fan Control.
I highly recommend these two apps. Give them a whirl and post any comments or questions here.