1) boot your Mac from your OSX install DVD
2) choose your language and then 'ignore' (cancel) the installation
3) from the Top Menu (Utilities) start Disk Utility (DU)
4) in DU click on your harddisk (not any partition) in the left pane (the first item listed)
5) in the right pane use the 'Partition'-Tab
6) Drag the partition separator line until it encompasses the entire drive and then select apply.
7) Quit Disk Utility
8) reboot your Mac from your harddisk.
This procedure only works with OSX 10.5 Leopard or 10.6 Snow Leopard.
Since you are 'fumbling' with your OSX partition you should consider having/making a backup of it before trying the a.m. procedure. Just in case.
Also, I believe Disk Utility can only expand an existing partition "downward" in its display, so it matters as to where on the disk the free space is located. If you start with two partitions, delete one of them, and want to expand the remaining partition to fill the free space, you can do it here, by dragging down the slider:
but you can't do it here:
I would be sure your disk is backed up before trying any of this.
There is an option.
I don't know if it's the best but it works for me.
Boot from a recovery disk/usb and open Disk Utility then:
1. Create a partition from the free space
2. Select the drive
3. Go to Restore tab
4. On the source drag the partition with your OS
5. On the Destination drag the partition you created
6. Click Restore
7. Go back to Partition tab and erase the OS partition (not the partition you created)
8. Resize the partition you create
I know this is an old topic but maybe someone is stuck with this problem and can't find an answer!