Mike Gravel is probably the most "libertarianish" choice in a Democrat (I think folks like him call themselves libertarian democrats), but not really close to Paul:
He claims to be "pro 2nd amendment," but wants gun licensing.
He's on the global warming legislation bandwagon, and wants gay marriage; no talk of getting rid of marriage licenses. He wants to fund social security instead of killing it, and also wants to "fully finance" the VA.
He wants "universal healthcare vouchers" (novel, I guess as libertarian as school vouchers, I.E., not particularly, but it recognizes the market choice is better than government workers, etc--I don't know why he doesn't have the same attitude toward the VA.) He wants a national initiative process--another novelty, which I have trouble supporting or criticizing on any basis except that it sounds as pragmatic (or not) as democracy itself.
He sounds similarly unprincipled to Paul on immigration but he's against the War on Drugs and the war in Iraq and/or Iran.
In short, he's the least harmful Democrat, but he's considerably less libertarian than Ron Paul. If he were elected President I'd consider it a move toward liberty in some areas, but not all. He seems the only truly anti-war, anti-drug-war Democrat. I wouldn't vote for him if Ron Paul wasn't an option (2nd amendment, taxation and spending are his obvious problems.) Finally, he doesn't appear to be nearly as popular, overall, as Ron Paul.