Just to point out
Q: Did you ad lib Han's famous line "I know"?
A: It's not really an ad lib, it was a suggestion, and movie making's a real collaborative process at its best. You don't ad lib it, you suggest it and then you try something in rehearsal and then you agree. So it was my suggestion, because I thought it was more of a character line than what was written, but the director and Carrie and I all thought it was a good idea at the time so we did it.
AMA with Harrison Ford
That said, the following are generally accepted to be unscripted lines.
"I can't see a thing in this helmet"
"Everything is under control. Situation normal. [What happened?] Uh... had a slight weapons malfunction. But, uh, everything's perfectly all right now. We're fine We're all fine here, now, thank you. How are you? [We're sending a squad up.] Uh, uh, negative. We had a reactor leak here now. Give us a few minutes to lock it down. Large leak... very dangerous."
Han Solo [with responses]
"So who talks first? You talk first? I talk first?"
"Quiet! And there's a fresh one if you mouth off again."
"I'll be there for you. The captain said I had to."
"Are you kidding me? I'm blind!"
It should be noted, the ad libbed nature of Han's conversation with the intercom in Episode 4 is because Ford deliberately failed to memorize those lines. He wanted it to feel more panicked and authentic. Presumably he had a general idea of what the script was, but obviously didn't follow it perfectly.
Its is "known" that Lucas dislikes improvisation and wants his actors to stick to the script. This could account for more ad libbed lines in the Original Trilogy than the Prequels (when he was a much bigger name). I could not find any definitive source for this assertion though. Perhaps it is telling that all intentionally on-camera improv was Ford, as Hamill and Fisher were both young and less likely to push pack.
Ford has spoken about how hard it was to get Lucas to change his mind about the script. Supposedly the "I know" line was Ford's idea and it was like pulling teeth to get Lucas to allow him to use it instead of the original line which, according to Lawrence Kasdan's original handwritten draft that he donated to the Writer's Guild of America, was "Just remember that, 'cause I'll be back".