Edit 2015-09-25: Don't remove simulators as suggested below. It may stop Xcode from running Playgrounds.
Xcode can eat up quite an amount of disk space. An easy fix may be to remove projects from the Projects window as well as remove Simulators. This only deletes data that can be recreated but as usual you're expected to keep a backup, like any professional does. Except Linus Torvalds, who is excused.
Before you do this, see the amount of disk space that's currently occupied by the Library folder with this Terminal command:
$ du -sm Library/ 27966 Library/
Take the following steps:
Now check the disk space again:
$ du -sm Library/ 25227 Library/
Nice, more than 2.5 gigs cleaned up :)