Partition after partition...
 

Primarily I assigned around 30GB to Ubuntu and 130GB to another OS. Now, I need more space for external memory computations in Ubuntu, and the amount I primarily allocated is not sufficient. Is there a way to "translate" some unused memory from the other OS to Ubuntu? It would be good not to everything from the beginning.
Thanks

Use Gparted or PartedMagic livecd and shrink the other partition. Make one more partition after it frees space. Let us suppose u have /dev/sda1 for ubuntu
and /dev/sda2 for the other os,
/dev/sda3 for swap

U can shrink /dev/sda2 and use the freed space to create one more partition as /dev/sda4. you can format /dev/sda4 and mount it to a mount point.

While Gparted can resize a partition it doesn't move the boot sector code which in any case would be in the wrong hard disk address once the partition left boundary is changed/moved. Thus one need to do a small bit of repair work with the boot loader.

Apart from the above it is not a big deal to resize a Linux partition that supported by Gparted.