Htop is a process manager that builds on the functionality of top. It allows you to view available memory and CPU usage as well as kill and manage running processes. It integrates quite well into the system manager Conky.
You can see it in action with this video via YouTube (credit: owburg):
Htop has a few features not found in top. First of all, the interface is more appealing, and it is easier to see what's going on. It opens a little faster, and you can kill processes by highlighting the name and pressing F9 instead of remembering the pid. You can scroll through the process list to see more than the first slew of items. Htop supports mouse operation as well.
As Debian Admin points out, top does have one advantage over htop:
[...] top is older, hence, more used and tested
sudo apt-get install htop
This is by far my favorite CLI process manager. In fact, I haven't used the GUI included in GNOME ever since I found this. If you need help with usage, don't hesitate to leave a comment or a link to your question on ubuntuforums.org.