Rave Master

Rave Master (called Groove Adventure Rave or simply Rave in Japan) is a previous manga of the author of Fairy Tail, Hiro Mashima which ran in Weekly Shonen Magazine from 1998 to 2005. Which tells the story of Haru Glory, a teenager who becomes the second Rave Master and is destined to bring peace to his world and save it from the threat of Dark Bring (Shadow Stones in the English version). Along the way he gains friends who join him on his quest and slowly unravels the mystery of the Rave Stones (his power source), The Dark Bring, his destiny, how it affects those around him and the fate of the world in general. In April 2011 Hiro Mashima released a crossover of Fairy Tail and Rave Master entitled Fairy Tail x Rave Special.

Many elements of this series pop up in Fairy Tail often. Many characters have similiar designs as Rave Master characters. Most notably:

Character Resemblance

 * Plue, Haru's guide, is one of Lucy's Celestial Spirits
 * Jellal Fernandes has the same facial tatoo and hair color as Sieg Hart. Jellal's thought project's alias, was Siegrain.
 * Etherion was in Rave Master as well, and it is a highly destructive Magic in both series
 * Natsu Dragneel resembles to Haru Glory, Rave Master´s main protagonist, in appearance, but also Let in fighting style. Besides, when Natsu dresses as Great Demon Lord Dragneel, his appearance resembles to Lucia Raregroove, an antagonist from Rave Master.
 * Gray Fullbuster has the same facial appearance as Musica, in Rave Master. The only detectable difference is the 3 spikes that is above Musica's eyebrows.
 * Erza Scarlet´s Requip Magic is very similar to Leopard (a member from the Blue Guardians, an antagonist group)´s Dark Bring ability, which consists in wear anything, even elements, and use it as armor, gaining its properties (i.e Fire: Burn, Wind: Speed increased and cutting ability).
 * Lisanna´s Take Over: Tiger appearance is almost identical to Leopard´s, which consists in a revealing cat themed suit with tiger stripes. Another resemblance is that hers supposed death is similar to Resha Valentine (a.k.a Elie)´s. Both were supposed to be dead at the start of hers respective stories, but later, is revealed that neither was dead, both were alive and well.
 * Cobra resembles to Shuda (an former enemy and later ally). Coincidentally, both characters were Oración Seis members in his respective mangas.

Rave Appearances
One can also spot characters from Rave Master in the background. The cover for chapter 91 even featured Lucy and Natsu dressed as Haru and Elie from the series.
 * Erza's armor has the same 'Heart Kreuz' logo as Elie's tank top.
 * Erza Knightwalker's Saint Spear the ten comandments transforms into Silfarion, Mel Force, Explosion, Gravity Core, Blue Crimson, and Rune Save, all with identical abilities.
 * Casino in Chapter 75 has a Ruby's figurehead.
 * The brand 'Heart Kreuz' is featured prominently (All Requip Amour has logo featured in some way or other, Lucy's Clothes, and both Erza and Lucy have Heart Kreuz Bags, Suitcases, etc.)
 * In the Omake Lucy's Photo shoot, Elie, Julia, Belnika, Musica, Ruby and possibly Griff (because it is of the same species as Griff and it has a perverted look on it's face) make a small appearance.
 * Griff makes appearances often, the most prominent being in the anime where Happy is chewing him instead of candy.
 * Griff explain Mest's memory Magic in chapter 215.
 * Grey waring the same white bringer in the cover for chapter 225.

For more information of the series, check the Wikipedia article on Rave Master or its Wikia site.