Erasing partition doesn't give me free diskspace
You will not be able to reclaim this space to your OS X partition unfortunately. That is because the Recovery HD partition is in between your OS X and (formerly) Ubuntu partitions. You have two options:


In the partitions tab, click the "+" and add a new partition. OS X will see this new partition as a separate drive, but it should be fully useable. However, it certainly complicates things vs. having a nice big single drive to work with.
Backup your Macintosh HD and wipe the entire hard drive (booting from another volume, such as Internet Recovery or Gparted on a disc). Then reinstall OS X, allowing it to properly recreate your partitions. This is more cumbersome but recommended.


When I partitioned my Mac for Ubuntu (using rEFIt) a while back, I ended up doing the second option to make my Mac "normal" again.