If you want a reliable cleaner, I suggest the one included in Ubuntu Tweak. Installation:
Originally Posted by jeshrel
Open in Applications > System Tools > Ubuntu Tweak.
sudo add-apt-repository ppa:tualatrix/ppa && sudo apt-get update
sudo apt-get install ubuntu-tweak
If you're quite familiar with Linux, you might also want to try BleachBit. It's very advanced and the interface and technical lingo might be confusing, but it's fully-featured.
Just a warning, though: not all of BleachBit cleaning options are safe. Most should be treated carefully and read thoroughly to avoid the possibility of unwanted file-deletion, as it will only prompt you once (or twice).
Installation: I believe BleachBit is already in the Ubuntu Software Center.