It's absolutely uncanny!
1: They were both introduced to mainstream audiences in the same movie, 10 Things I Hate About You. Ledger had the lead, so of course he got all the recognition, but Gordon-Levitt didn't do too badly afterwards either.
2: Following that hit, they both made the conscious and radical decision to be serious actors and not star in big hits all the time. Ledger dabbled in blockbuster territory with A Knights Tale and The Patriot, but then opted to do smaller films, like Monster's Ball. Gordon-Levitt wasn't even seen by the mainstream again until G.I. Joe and (500) Days of Summer last year, instead making small, indie gems like Brick, Mysterious Skin and The Lookout for a good 10 years.
3: They both had their big coming out party courtesy of Christopher Nolan. Ledger had his in The Dark Knight, and Gordon-Levitt, Inception. Both were already climbing the ladder to stardom, but they each jumped most of the rungs to land on the top with those respective films. Yes, I am aware that Gordon-Levitt was a series regular for most of the run of 3rd Rock From the Sun, but that's TV, and I don't care, and Ledger got his first Oscar nom and heaps of critical praise for Brokeback Mountain, but that film was too controversial to appeal to a good chunk of the demographic that flocked to TDK, so he wasn't as much of a household name at that point.
4: Another Christopher Nolan similarity for you. God willing, they will both play the main villain in their own installments of Nolan's Batman franchise. Ledger won the Oscar for perfecting the character of The Joker, and Gordon-Levitt looks like the clear favorite to play the green clad trickster, Edward Nigma/The Riddler in Nolan's upcoming sequel to TDK. Please let that happen! I will never ask for anything ever again is JGL is The Riddler!
5: Neither one of them let(s) stardom get to their heads. Ledger always kept a cool demeanor, and didn't just take a role because it would have paid him a lot. I mean, he followed the most profitable, non James Cameron movie ever with a Terry Gilliam project that no one saw. Gordon-Levitt is more of the same. He did Inception, and what does he have coming next? An indie film, Hesher. The both take/took roles because of the artistic merit behind them, not because of the paycheck they would be receiving.
6: They look kinda similar, so... there's that.
Well? What say you? Am I on to something or am I so far off the mark that I should just go jump off a bridge right now? Well, whatever. I think it's interesting to point out. Smiley face emoticon!
Next week, I'll show you how Zooey Deschanel and Katy Perry are actually the same person!
Ha ha ha ha ha! No!