Messages works on my iPhone and iMac, but not on my iPad.

Here's the troubleshooter for this:

iOS: Troubleshooting FaceTime and iMessage activation

Using FaceTime http://support.apple.com/kb/ht4319

Troubleshooting FaceTime http://support.apple.com/kb/TS3367

The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting

http://tinyurl.com/a7odey8

Troubleshooting FaceTime and iMessage activation

http://support.apple.com/kb/TS4268

Using FaceTime and iMessage behind a firewall

http://support.apple.com/kb/HT4245

iOS: About Messages

http://support.apple.com/kb/HT3529

Set up iMessage

http://www.apple.com/ca/ios/messages/

Troubleshooting Messages

http://support.apple.com/kb/TS2755

Setting Up Multiple iOS Devices for iMessage and Facetime

http://macmost.com/setting-up-multiple-ios-devices-for-messages-and-facetime.htm l

FaceTime and iMessage not accepting Apple ID password

http://www.ilounge.com/index.php/articles/comments/facetime-and-imessage-not-acc epting-apple-id-password/

Unable to use FaceTime and iMessage with my apple ID

https://discussions.apple.com/thread/4649373?tstart=90

For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8

 Cheers, Tom 😉