There's Psensor which can monitor the motherboard and CPU temperatures, the temperature of Nvidia GPUs, hard disk drives temperature, fan speed and CPU usage. It can also display hot temperature alerts using desktop notifications and the application indicator which turns read when a threshold is reached. The alerts are not enabled by default though and have to be enabled.
T o install it, firstly you need to install lm-sensors and hddtemp as these packages are required for Psensor to be able to monitor CPU, hard disk and motherboard temperatures as well as CPU fan speed:
After that install Psensor itself:
sudo apt-get install lm-sensors hddtemp
sudo service module-init-tools start
sudo add-apt-repository ppa:jfi/ppa
sudo apt-get update
sudo apt-get install psensor