Enter this to see the current partition usage:
df

E.g.
Filesystem 1K-blocks Used Available Use% Mounted on
rootfs 3698504 3351472 146524 96% /
/dev/root 3698504 3351472 146524 96% /
devtmpfs 494800 0 494800 0% /dev
tmpfs 99820 240 99580 1% /run
tmpfs 5120 0 5120 0% /run/lock
tmpfs 199620 0 199620 0% /run/shm
/dev/mmcblk0p1 57288 19400 37888 34% /boot

The rootfs and /dev/root is the same thing, its just a debian thing the have them both appear as separate

Enter this:
sudo fdisk /dev/mmcblk0

Then press 'p' to see the current partitions on the disk. E.g.

Disk /dev/mmcblk0: 7860 MB, 7860125696 bytes
4 heads, 16 sectors/track, 239872 cylinders, total 15351808 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xa6202af7
Device Boot Start End Blocks Id System
/dev/mmcblk0p1 8192 122879 57344 c W95 FAT32 (LBA)
/dev/mmcblk0p2 122880 15351807 7614464 83 Linux

Now delete the 2nd partition (it won't actually delete the data on it)
Press 'd' > Enter
Press '2' > Enter

Now re-create it:
Press 'n' > Enter
Press 'p' > Enter
Press '2' > Enter

Enter the First sector and the same value as the original /dev/mmcblk0p2 partition (122880 from the above example).

For the Last sector just press enter to use the maximum value.

Now press 'p' > Enter to see the new partition setup.
Finally press 'w' > Enter to write it

Now reboot:
sudo shutdown -r now

Once its back do the resize:
sudo resize2fs /dev/mmcblk0p2

This may take a little while to complete…

Now enter the command
df

and hey presto, you've expanded to use all the space ?