If you have regularly used iTunes to purchase music and movies, then you may have a significant collection of titles that are tied to your specific iTunes account. However, if you have used more than one iTunes account, then you may run into trouble if you cannot remember the account ID you used to purchase the items.
At the very least, if the songs are DRM-protected and you have not authorized your system to play them, then iTunes will present you with a notice to log in to your account before it will play them.
While Apple offers a quick password recovery link in the authorization window, this requires you first enter a valid corresponding Apple ID, so if you have forgotten your iTunes account ID, then you may be at somewhat of a loss for what to enter. However, there is an easy way to look this up using the items you have already purchased.
As shown in this Apple knowledgebase article, each song or movie you purchase in iTunes will be tagged with your name and the account used, so you can use this to find your lost account username.
To view this information, simply select any song or movie and press Command-I or choose "Get Info" from the File menu. Then in the Summary tab of the window that appears (this should be the default tab), you should see the name and Apple ID listed. Now you can use this Apple ID as the username in the iTunes authorization window, and then retrieve the password as well if necessary.