About 30 years ago I had a boss (who I liked) ask me for 7 weeks more work, after I gave notice. I gave him the 7 weeks (new company was OK with that; I had worked for them before), even though it was a personal hassle. Fast forward to 5 years ago: same ex-boss and his wife visited my wife and I when they were traveling through Arizona, and we had a great time.

That said, now in most cases, everything is at-will and neither side really owes the other anything, although it is good to be polite and not burn bridges.