Kid Cudi. His recent album, Man on the Moon, features indie acts like MGMT and Ratatat and has a really eclectic sound. Try "Pursuit of Happiness" and "Make Her Say," (featuring Kanye West and Common) which samples Lady Gaga's "Poker Face."

Have you heard Girl Talk? He's a DJ who mashes up hip-hop samples with hits from the '80s and '90s. He has several albums (you can download songs from his most recent, Night Ripper, here for free) which are all amazing. A lot of people do the mash-up thing, but Girl Talk is by far the best. (Listen to the greatness here, and get ready to jump out of your seat.)

Your crush might like Broken Bells, which is the new album from Gnarls Barkley member Danger Mouse and James Mercer of The Shins. It's a must for fans of Gorillaz, another group your guy might like.

What about old-school hip-hop groups like Tribe Called Quest and Wu-Tang Clan, or newer groups and artists with an old-school sound (The Roots, The Fugees, Jurassic 5, Mos Def, Talib Kweli, Lupe Fiasco)? These artists are perfect for someone new to hip-hop .

You should also be able to find The Grey Album-- the album where Danger Mouse mashes up Jay-Z's The Black Album and The Beatles White Album-- floating around online. It's really fantastic. Plus, it could get you into Jay-Z. (Listen to The Blueprint if you want your mind blown.) 

Speaking of Jay-Z, Collision Course, his live album with Linkin Park, is actually pretty great. (You've probably heard the "Numb/Encore" mash-up.) Jay-Z makes everything better.

As for indie/alt stuff, he might like Passion Pit and Crystal Castles. Both bands are high energy, danceable, and a ton of fun. (Anamanaguchi, who make catchy music using old Nintendos and other video game systems, are also great.) Also, I know a lot of hip-hop fans who are into bands like TV on the Radio, LCD Soundsystem, and Arcade Fire.

(And of course there's those great musical equalizers, Beastie Boys, Beck, and Prince. I don't think there's a person alive who doesn't like at least one of song by these three artists.)

What about "Airplanes," that song by rapper B.o.B. featuring the singer from Paramore? That's a solid jam. Lil Wayne's recent album is hit-and-miss, but also pretty rock-heavy. I like "Knockout" and "Drop the World," featuring Eminem.

Do you like M.I.A.? V.V. Brown? Kid Sister? Janelle Monae? The ladies are killing it right now in hip-hop and R&B.  

Also, you can't go wrong with Kanye West. I don't care what people think about him personally-- the guy make some seriously awesome music. His stuff is soulful, filled with great samples, and musical in the best possible sense of the word. Late Registration is probably my favorite album, but I also love Graduation and 808s and Heartbreaks. I'm currently playing out his new single "Power."

Hope this helps! Any other suggestions?