Artificial light has to be very close to the plants to make a difference. A regular shop light with fluorescent bulbs suspended an inch or two above the seedlings would work.

If you're starting tomato seeds in a window, the plants may get leggy. When it's time to plant them outside, plant them deeply in the ground up to the top set of leaves and the stem will grow roots. You can only do this with tomatoes and not other plants.

" It gets quite a bit of sunlight, but I figure it can't hurt to add some artificial as well. The ceiling has a single bulb light fixture."

If your porch faces South, you can boost the light to the seedlings with a reflector behind them.

Cardboard faced with shiny Aluminum foil will do fine.

Just set it behind the plants so they get the benefit of light from both sides and watch them a while ... you don't want them getting too hot.

In other words, don't box the plants in completely.

As for artificial light, there are some High Intensity Discharge lights you could buy.

I don't know jack about them, though.

And they are unlikely to fit your regular light socket: even if they did, it would almost certainly be too far away from the plants to do any good.

You could run an extension cord to your porch, right?

There's a screw in plug that converts a light socket to an AC outlet, but don't go crazy with that, your porch light circuit might not be rated for huge amounts of power.

You could also go with regular florescent lights, behind the plants so they don't block the natural light. Some florescents can be 'overdriven' to provide an extra 50 - 70 percent of light.