I started noticing a foul smell in the room on the side where that pipe is, and when I went out to investigate it, there was a nice size pile of sewage (feces, toilet paper, etc.) right around the area where this pipe coming out of the side of the house (almost at the base of the house, I don't have a basement). I think this is an overflow/clean out pipe, but I am not completely sure. It use to have a cap on it, but now I don't see one. I flushed the toilet and watched this pipe and water came out of it as soon as the water went down in the toilet. What could have caused this cap to come off? Should the water and everything else from my toilet be coming through this pipe even though there is no cap, or is there a clog somewhere else that is forcing it out this way? What is the best solution to my problem (and least expensive)?

I have no idea how the cap came off your clean out, but it should have one for many different reasons, one being to keep sewage from being pumped out on the ground, another is so that a small pet or rodent can't get into the pipe and cause problems.
The water coming out of the clean out means that your sewer line is stopped up somewhere between the clean out and where it connects to the municiple sewer line. You should have a plumber clean out this line.
He will probably have a replacement cap or plug with him.

Yes it needs cleaned out, but it would be a lot cheaper to rent a big sewer auger and do it yourself. If you do hirer somebody, go with sewer cleaning specilist rather than a plumber. When you are done, pick up a cap at a hardware or home center. Don't cement it on. Likely the sewage forced the cap off. Look around, it may not have gone far.

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Yes it needs cleaned out, but it would be a lot cheaper to rent a big sewer auger and do it yourself.

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I have to disagree with this suggestion.

If you have never used one before and haven't been properly instructed in it's use, then you have no business using one.

Period.

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If you do hirer somebody, go with sewer cleaning specilist rather than a plumber.

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Again, I disagree.

Most Plumbers are well versed in the operation of "sewer augers" and are generally a lot more inexpensive to hire for this purpose than a "sewer cleaning <sic> specilist".

I disagree. Operating a sewer machine by an inexperienced person is begging for trouble. You can get the cable stuck, or broken off, or even do serious injury to yourself.
You can also splash 'stuff' on yourself [keep mouth closed when operating sewer equipment].

"you can also splash 'stuff' on yourself [keep mouth closed when operating sewer equipment."

Hey! That reminds me of the plumbers creed.

COLD WATERS ON THE RIGHT
SH-T WON'T RUN UPHILL
PAYDAY'S ON FRIDAY
THE BOSS'S SONABI-CH
AND- NEVER-NEVER BITE YOUR FINGERNAILS AFTER YOU SNAKE A SEWER

Wow, thanks for posting this. I'm just very glad that I have good <a href="http://www.westvac.com/?page_id=34">sewer equipment</a>, so that I can stop something like this before it happens. I hope you get this all sorted out, and soon!