Best Computer Cleaners For Free
CCleaner is one of the safest cleaners without any nag screens or offers, just an option in the window for an upgrade that is discrete. Glary Utilities is more thorough, but that also means more chances of errors. IOBit Advanced System Care free is very good and nags you to upgrade if you choose an option that is for pro only, but otherwise doesn't nag you. I use CCleaner and Advanced System Care Pro daily, and once in a while I use Glary Utilities. If you want a good and free memory cleaner, CleanMem works great and is a set it and forget it program. It creates a task to run every 15 minutes to reduce memory usage that programs take but doesn't give back when it doesn't need it (programs use more memory when it opens, but doesn't usually need as much after it is running).
Best Free Computer Cleaner For Mac
Ccleaner
Hi Samarth, For Windows, there are a few great tools: Ccleaner - Lavasoft AdAware Free - Malwarebytes Free - Iobit Advanced System Care - Bear in mind that these aren't all quite as you described, and may ask you if you want to buy a more complete version. However, they WON'T require any money or upgrades to do their job, and that is the job of cleaning your PC of malware and other crap you don't need lying around. When it comes to malware and spyware removal, you can't really have enough tools - these are just a selection of my past favourites which I pulled off our Best Windows Software page. You should check it out, it's a complete list of freebies for Windows and there are a few more tools (like Defraggler and anti-virus programs like Avast) which is must-have software. 
Good luck Tim.