Here's one odd effect a government shutdown would have: NASA would likely have to scrub the launch of space shuttle Endeavour's STS-134 mission, currently scheduled for April 29. If Republicans and Democrats cannot agree to budget terms by midnight tonight, Washington will effectively run out of money and the government will close.
If that happens, according to a NASA memo distributed today, only operations critical to protect life and assets would continue. So, operations to support the astronauts on the International Space Station would go on during a shutdown, as would any operations critical to prevent the loss or damage of NASA assets. And if a launch were in progress when the shutdown went into effect, that launch would continue. But for new shuttle launches, and other new projects: an indefinite delay.
Memo (PDF) and NASA furlough plan here (thanks, Miles O'Brien!).