Is there any software / application to clean the unused / temp files on the phone memory of Nokia N8? Like we use C Cleaner for windows on PC.
You could try Netqin Mobile Guard for Symbian available from HERE.
Not only does it clean spam from your phone it also has a few other useful functions.
To manually free up space from your C: drive you can regularly clear the browsers cache, empty your SMS folders (inbox and sent) and remove emails as well.
I am using fsecure. I have heard that installing netqin will slow down the phone. As well as I regularly clear all the cache files. But I think that, there must be some temp files remaining when we install/uninstall apps.