Acting for Two: As with any animated series, you'll find some of this among the minor and one-off characters, but the most notable instance is Sean Hennigan. His main role is Decade, but he also plays the mob boss from November 11's introduction at the beginning of episode 5. This may have been a deliberate Casting Gag since the mob boss and his men are the first of November 11's victims we see, and Decade and his men are the last.
Actor Allusion: April's Japanese voice actress dubs many Halle Berry characters, including Storm of X-Men, with whom she shares weather-control powers and appearance.
For Hei: Chinese Electric Batman, for pretty obvious reasons. In the second season, with the addition of Wild Hair and Perma Stubble, he has been dubbed "Hobo Hei." Or "Heibo", if you're one of those informal types.
In fact there's a lot of D.Gray-Man voices in this one: July is Road Kamelot, Amagiri is Kanda Yu, Havoc is Lenalee and the entire Police department except for Matsumoto are voiced by people who appeared in the series (including the major characters of Bookman, Cross Marian and main character Allen Walker) in the English version. In the Japanese, Kanami Ishizaki is voiced by Allen's voice actor. It's probably harder to find characters in this series that have voice actors no way related to DGM from the sheer number.
A lot of this is because the English dub is by FUNimation, who tend to use a few dozen voice actors most of the time. Careful listeners may also notice a decent portion of the English casts of Baccano!note In particular, Jason Liebrecht as Luck Gandor sounds exactly like Jason Liebrecht as Hei, except with a New York accent, and Todd Haberkorn's Yutaka Kouno could pretty much be Firo Prochainezo's grandson and Fullmetal Alchemist.
November 11th is also voiced by Kazuhiko Inoue, who also voices a certain copy ninja from Naruto...
Also pretty much the entire female cast of Claymore makes an appearance.