As songs go for beginners, "Smoke on the Water" is a good one but I gotta say, the song has been ruined for me with hearing it on the 'beginner' guitar so much.
Here's a list of some easy to play beginner songs that offer a little variety.
There are a lot of Beatles songs here, but actually for as popular and creative as the Beatles were, their songs are quite easy to learn --- if you like the Beatles that is
Bad Moon Risin' (Creedence Clearwater Revival)
California Dreaming (Mama's and the Papa's)
As Tears Go By (The Rolling Stones)
Daydream Believer (The Monkees)
Dock of the Bay (Otis Redding)
Eight Days a Week (The Beatles)
Have You Ever Seen the Rain (CCR)
I Saw Her Standing There (The Beatles)
I Should Have Known Better (The Beatles)
I've Just Seen a Face (The Beatles)
Mellow Yellow (Donovan)
Midnight Special (CCR)
Nowhere Man (The Beatles)
Ob-La-Di Ob-La-Da (The Beatles)