Depending on the dimensions of your tank, probably some sort of T12, T8 or T5 shop light.

I use clip lamps (brooder lamps, work lamps, whatever they're called) with daylight balanced CFLs which can work in your situation but may not be ideal, based on the dimensions of your tank. I like them because I can position them at different distances and get different lighting situations within the same tank. Also, CFLs are easier for me to deal with than tubes. The only tubes I am using are T15s for my African violet shelves. Two things to think about: how are you mounting your lighting system? and does your tank have a lid? My one case has a hinged lid on top, and I am clipping lamps off of a wire shelf and a floor lamp nearby, which effectively means that I will always have to have those pieces of furniture nearby. Or find a good substitute. My other terrarium is a reptile terrarium that has doors that open in front, so I just lay the lamps on top. This probably does up the temps within the tank somewhat, but it doesn't seem to be a problem so far.

Thanks for the responses!

My thought is to put either a screen or plexi-glass top on it and set the lights on it. Will the light be fine going through plexigass? Only thing then is I won't be able to adjust the proximity of the lights to the plants...

Acrylic (plexiglass) will sag, especially under warm lights. You could buy some thicker acrylic, but that can get expensive. I'd find a glass shop and see how much they'd charge you to cut a couple pieces to size. I did that for a 40g tank I used to have. The local fish store may have those stick on handles for glass aquarium covers, or you could have the glass shop drill some small holes for a wire handle or even just a finger hole.

As for the lights, T8's would be cheap and provide good coverage. I believe a 55g is 12" wide, so if you can find fixtures that are 6" or less, you could get 2 fixtures (4 bulbs) on there. If you go this route, try to get fixtures with good reflectors that curve around the bulbs as much as possible so that you get more light focused down on the plants where it's needed. For bulbs I like to use 6500K (daylight). Cool whites are also reasonably close to daylight.

Plexiglas has a maximum transmittance of 92% of perpendicular striking light. Actual transmittance from a point source will be much less. This may or may not be a concern depending on the lights and the plants you choose.

Plexiglas is also opaque to infrared from 2800nm to 25000nm. In combination with reduced airflow caused by the lights resting directly on the surface of the plexiglas, this might cause overheating or reduce the lifespan of your lights and ballasts. This all depends on the lights you choose though, high output lights tend to generate more heat and would probably suffer more from reduced circulation than lower output lights.

A better solution might be to hang the lights or rest them on a frame rather than a solid pane of plastic.

Source: plexiglas.com, Plexiglas optical and transmission characteristics