If you're looking for manga similar to 21st Century Boys, you might like these titles. All recommendations are made by Anime-Planet users like you!
Kirihito Osanai is a talented doctor with a successful career and a beautiful fiancée. However, once he's sent to a remote village to investigate a rare disease known as Monmow, his life quickly changes forever. After spending time with the infected patient, Kirihito begins to show the symptoms of the deadly disease - his body physically changes to a dog-like form and he begins to crave raw meat; what awaits him is a slow and painful death. Thus begins Kirihito's nightmarish journey into the unknown. He's kidnapped, forced to perform as a circus freak and branded a deviant by everyone he comes across, while back home in Japan his ex-colleagues and fiancée search for what happened to him and uncover a conspiracy that shakes them to the core. Meanwhile, the search for the cause of the Monmow disease continues...
Ode to Kirihito and 20th (21st being the conclusion) aren't similar in plot, but I definitely felt similar after reading them (NOTE: 20th/21st are FAR better and way more epic than Ode, but Ode is also good). Call it a gut feeling, but I think you'll appreciate the similar storytelling style.
Fifteen years ago, Garai was part of a group of delinquents who kidnapped Yuki, the young brother of a famous kabuki actor, on a remote island which housed a secret military base. Garai couldn't control his lust for the young boy's girlish looks and took advantage of him in a cave, followed by a grim discovery: everyone on the base was dead from an apparent leak of MW, a powerful neurotoxin. In the present, Yuki is alive, but exposure to the MW destroyed his conscience; he now lives a double life as both an attractive bank employee and a sadistic, brutal killer who kidnaps, rapes and destroys others for his own whims. Garai, plagued with guilt, has become a priest and tries desperately to save Yuki's soul, though often ending up in bed with him instead. Yuki's ultimate goal is to find MW once more to destroy the world; can Garai stop him in time?
Not really anything compares to 20th Century Boys (21st Century Boys being the conclusion of it), but MW is close. Both are gripping tales of terrorism, drama, and more - and are told in a very VERY similar way. Unsurprising as Urasawa seems to greatly respect Tezuka (re: Pluto). If you liked one definitely try out the other, though MW is only a fraction of 20th's awesomeness.