What's Training? is an addon for World of Warcraft Classic that adds a list of available trainer spells to the spell book, so you can easily know when to seek a class trainer out. Spells you already know are moved to the bottom. Mousing over a spell will show you its tooltip, with the cost to train attached to the bottom. Mousing over a category header will show you the total cost of training of spells in that section.
NEW IN 4.0:
WotLK support
If you have Class Trainer Plus installed, What's Training will show Ignored spells at the bottom.