I have been playing with my Raspberry Pi for couple of weeks now and recently I ran into this issue of low space. Even though I had 8GB memory card it was only partitioned to use 4GB. After Googling for sometime I found there is a built in tool with Raspberry Pi which helps to resize the partition easily ie raspi-config.
Please check if your Raspberry Pi is running on Debian GNU/Linux wheezy/sid
root@raspberrypi:~# cat /etc/*-release
PRETTY_NAME="Debian GNU/Linux wheezy/sid"
# sudo raspi-config
raspi-config will configure Raspberry Pi to resize the partition during next reboot
raspi-config will request to reboot
During reboot process you can see that partition is getting resized
Once reboot is completed you can see more space on your rootfs.
I think this is the best way to resize the partition. No data loss or any special tool required to complete this operation 😀