Can I play music from my macbook on my iPad for free?
This is exactly what iTunes Home Sharing allows you to do -- stream music stored on my Mac using your iDevice.

The short steps for setting this up are:


Enable home sharing in iTunes on your Mac:


Start iTunes
Go to Preferences -> Sharing and make certain 'Share my library on my local network' is checked. 
LEAVE ITUNES RUNNING!

Now connect to your shared library from your iDevice:


Go to the Settings app and click on the Music settings
Scroll down to the bottom of the Music settings page and you'll see the 'Home Sharing' setting
Enter your iTunes account ID and password to enable your iDevice to see your shared library

Use the Music app on your iDevice to access your shared library


Open the Music app
Your shared iTunes library should now be available as a source of audio in the app 



This works for video, audio and podcasts as well. If your Mac puts itself to sleep you might need to wake it back up for the shared library to show up in your iDevice before you can stream audio or video from it.