The question has been raised numerous times since his death. Well here, with a HitEmUp exclusive, we're going to break down the facts for you. I know webmasters out there are going to be biting this. So for the webmasters, give credit where it's due, and to the fans, any sites that don't credit are fake and shouldn't be supported.
His Marriage to Keisha
If you listen to Tupac's earlier work, you won't find any mentions of his wanting to have children, however around the time of Me Against the World you can start to notice this desire emerge. At the time he was with Keisha Morris, who he proposed to after a 3 month relationship. In a recent interview with Source magazine Keisha Morris stated that she and Tupac had attempted to have a child. She stated that the amount of weed Tupac smoked may have prevented this. When Tupac was released, with the stress of litigation, being shot and his work load, their relationship deteriorated.
Mentions in Song
There are mentions of a child in tracks as far back as album number 4. If you listen to the intro of Death Around the Corner on the album Me Against The World, you will hear this.

"Child: Why you by the window?, what's wrong daddy?
Mother: I know what's wrong with that crazy .."
Rather odd, but Tupac often mentioned his relatives children has his own.
Also on Me Against the World on So Many Tears he says "Disillusioned lately I been really wantin' babies, so I can see a part of me that wasn't always shady"
There are many mentions in songs that would lead you to believe that Tupac didn't want to have a child. In Smile he raps: "Somebody save me, Lost and crazy, Scared to drop a seed, Hopin' I ain't cursed my babies"
and in Hell 4 a Huster: "no baby mama drama ... why plant seeds in a dirty b*tch waiting to trick me, it's not a life for me."
In the track Cause I Had To Tupac says "pop at me, brand me, shoot, but what the fuck can a nigga do? my little boy gotta eat too."

Mentions in Song
If you listen to the unreleased song Words to my Firstborn Tupac says: "Since my very first day on this earth, I was cursed, so I knew the birth of a child would make my life worse, though it hurt me, there was no remorse, cause wild seeds can't grow, watch your soul"

and later says: "all I could give it was my debt and my last name, cause in the game things change, living up and down, this hard life got me walking with my head down, flashing frowns, wasn't meant to be was I wrong ?, but I'll never get to know so I carry on"
This song is rather odd, titled Words to my Firstborn he basically writes it for the child he didn't have.
Despite the fact that Tupac felt his lifestyle made it unfair to bring a child into the world, there was definitely a side of Pac that yearned to settle down and have a family. The older Tupac got the more obvious this became. In Do for Love which became public after his death he raps: "You tried to trap me say you pregnant, and guess who the daddy ? Don't wanna fall for it, but in this case what could I do?"
This is a pretty subtle reference, as the song may have been written hypothetically, however in the song Unconditional Luv Tupac raps: "While getting paid, At the same time, just had a baby with the same mind, Something inside, Please let me die, These are strange times"
This verse sounds pretty straight forward. Nowhere, in any song, is Tupac's desire to settle down more evident than in the song Happy Home. He sings: "We promise to be good, To each other, Plus I love you, So I know You goin' make A good mother"
"Born Through hard times, Ghetto child of mine, I wonder, If you have to suffer, For your father's crimes"

"Though Look at him, Walkin' and talkin', A little child With my eyes And mouth, Father Watch over little seeds, Help me Guide them out, Had to change My whole lifestyle, Married my Baby's momma, Made her My wife now, I'm tryin' hard y'all, Maybe in time I'll be a better man, Watchin' the older couples Handle it Like veterans, Show me the meanin' Of forever, And together we rise, If it would, Help our child grow"

"Question my love, It's so easy to see, Without my family All I'm left with Is a shadow of me."
Tupac and Keisha were trying to have kids before he was imprisoned in 1994. In Just Like Daddy Tupac appears to mention Keisha taking care of the kids during this time:
"You stayed locked down at mom's house watching the kids through the whole bid"

Conclusion
If Tupac had a child, he wanted to call it Star if it was female, or Michelangelo if it was male. Whether or not Tupac had a child is a very serious question; as of now there is nobody to carry on his legacy. The quotes in these songs would make it seem as though Tupac did plant a seed. But if he had a child, would they want to carry what Tupac called "the burden" of that legacy ? As time passes and the world and people age, we may find the answer to this question.