Q: I started an account on iTunes when iPod first came available. Bought several iPods in that time. Eventually I changed my email address and continued to buy music and games.  Now I have 2 Apple ID. How can I get everything on one ID?

The short answer is you can't.  Apple ID's cannot be merged.  As long as you have the password for both accounts, you can re-download the content to a computer if it is lost.  The best solution is to pick the one with the most content associated, and use that ID. 

HTH.

http://support.apple.com/kb/HT5622?viewlocale=en_US

-The specific point:

Apple IDs cannot be merged. You should use your preferred Apple ID from now on, but you can still access your purchased items such as music, movies, or software using your other Apple IDs.

If you are wondering how using multiple Apple IDs relate to iCloud, see Apple IDs and iCloud.