I had to ride in the pouring rain through lots of puddles for about 30minutes the other day. I laid my bike down yesterday and water came out... What maintenance do I need to do on it to get rid of the water, and prevent rusting of the inside of the frame?

Might be too late for framesaver..
I think you may be over worried however - just dry out the frame the best you can

Water does not rust bikes and riding in the rain isn't that big of a deal...

Just don't leave your bike outside overnight and dry the bike with a rag then relube your chain.

Steel is a very resilient material and will hold up for decades despite the formation of rust. There are some cases, however, where bikes undergo premature failure due to excessive exposure to corrosive elements such as road salt.

In the case of the occasional rain ride, however, I would just do what Squirrelli said. Dry the bike off to the best of your ability and make sure your chain is lubricated. It's also a good idea to put some sort of anti-sieze component in your seat tube to prevent your seat post from binding with the seat tube. Framesaver is also a product you can spray into your frame to prevent rust.

Where are you guys getting that water does not contribute to the oxidation of any ferrous alloy? Its fairly common knowledge. Bikes frames are not mythical creations immune to rust. Moisture in the air, and exposure to water will accelerate the oxidation of iron based alloys. It may take a while for it to actually affect the structural integrity of the frame, but it will still happen. Basic chemistry.

Throw your bike into a fire and let all the moisture evaporate from inside the frame.

I'm not worried about rust, but if I had a steel bike frame that was my favorite and wanted to keep forever, I would take it to a machine shop and give it an acid bath. After the frame is clean and clear of rust, protect the inside with linseed oil, and the outside with chrome/paint. That would be enough protection for the Amazonian Rainforest.

Which he doesn't need because, beyond surface rust, frames don't rust through from occasional contact with water. Unless they've been exposed to something corrosive or left in standing water for an extended period of time, the OP really doesn't have anything to worry about.

if you don't normally ride in the rain, i'd wipe the bike down with warm soapy water and a sponge, paying attention to the underside and making sure no dirt was accumulating. i might hit it with some spray car wax after that (basically wash it like a car). then check the chain and all of the axles for signs of water intrusion

if it's just a beater, dry it off and make sure the chain is lubed

Maybe if I move to Portland

Nah I practically live there, it never rains

In all honesty though I own an old ass schwinn letour with its entire life span of being rode has been in the northwest and has sat outside for years, I weight close to 200lbs and it holds up just fine still to this day so I wouldn't worry TOO much.. probably actually not at all