Face the facts, Libya is in a civil war. The 'rebels' have gained control of most of the country, and are moving forward to take all of it.
There have been mixed reports on whether they have asked for international help. Some suggest they've asked for a no fly zone and/or weapons, others that they've said they don't need help, and even a few saying they've asked for international troops to join them.
For NATO, the US, and the rest sending troops in would at best just be a waste of resources. Most likely it would result in casualties to already stressed militaries. Worst case they would be seen as invaders, by both sides and end up fighting a 3 way war. One thing for sure, Qaddafi would use them as a way to claim that the rebels were being run by the outside world, likely the US in particular.
As for giving them weapons, this is another bad idea. One the US has run into time and time again; help one side in a civil war, then end up fighting against those same weapons (if not the same people) in another war.
Now a no fly zone, could have real merit, and in my opinion is the best of the 'official' options. Tis a lot more realistic of a goal, with minimal ground troop usage and risk to the military forces involved. However it does mean taking a definite side in the conflict, something the Obama administration has been very reluctant to do. There is also that the US would once again be 'flexing its militray might' at the world. Something which even when done for the right reasons, causes the US negative reactions from the world.
The option of sitting back and doing nothing is the most likely, for most of the world, and especially the US. Definitely has the least risks, and the outcome is all but certain anyways. But it does mean standing by whilst hundreds, maybe thousands more, are killed, which to me does not seem the right thing to do.
My personal suggestion: Eliminate Qaddafi, a known mass murderer, who has committed or ordered who knows how many crimes against his own people and humanity. And then let the people of Libya work things out for themselves.
Benefits to this plan: Civil war likely almost immediately ended, lots of lives saved and damages to infrastructure lowered, and lowest risk to international troops.
Possible issues: Power vacuum may occur, but unlikely as the rebels already have set up a government. US (or whoever does it) seen as the evil empire again, nothing new there though. And this time could be done publicly.