And on the subject of rap-like music, stormfeather mentioned this before but i haven't. Some group or another did "Roll a D6", a D&D parody of "Like a G6". And someone else put together "Damn It Feels Good To Be A Lannister". Warning though, that does contain some spoilers if you haven't read or seen "A Song of Ice and Fire"/"A Game of Thrones." And if you've read the books but are still planning to see the show i'd still avoid it until afterwards since it uses one or two bits that were actually kind of surprising. If you haven't partaken of either and never plan to do so, you can go ahead and watch it but it will make absolutely no sense for you :) But in any case, clearly i appreciate rap a lot more when it's in the form of geeky parodies of rap music.
Off of the subject of rap, these people made a fun, silly, dorky Transformers parody song and then followed it up with a Thor parody song. They're definitely not as musically impressive as a lot of the others videos listed here, but like i said, they're very silly and dorky :)
And finally, some parodies of a song i _do_ like the original of:
X-Men - Born This Way and...
Minecraft - Form This Way
And as long as i'm linking videos, any gaming geeks who haven't seen the promotional "The Guild" music videos yet should check them out. I'd never seen any episodes before i watched the videos (and have only seen two or three episodes now) but i still thought they were great.
The Guild - Do You Wanna Date My Avatar for season 3 and...
The Guild - Game On for season 4. Sadly they don't seem to have made one for season 5, at least not yet.