The most obvious example of a group of people using clothing as a statement and form of personal expression are the sans-culotes of the French Revolution, who neglected the fashionable knee-breeches of the nobility in favor of practical, workmanlike pants. The distinctive dress of the sans-culotes served as a political statement, a uniform, and a form of resistance.

If that doesn't answer your question, let me know, I have other examples if you can be more specific as to what you want to know.

For further reading: The Sans-culottes by Albert Sobul

I said I would come back! I just didn't actually get home last night.

Ok, onward!

What you're asking covers almost all of history and all regions. I'll try to give you some really basic examples that give you an idea of just the kinds of things people would do for their own clothing.

The hardest thing about finding decorative clothing worn by peasants is that their pieces were generally not preserved and they weren't painted as often. I'll start by linking to some paintings of peasants throughout history. Here is a painting by Peter Bruegel from the 16th Century: http://upload.wikimedia.org/wikipedia/commons/c/cd/Pieter_Bruegel_de_Oude_-De_bruiloft_dans(Detroit).jpg . The most noticeable thing about this clothing is the variety of colors. Here is another image of peasants from about 1412 in a similar palette. There was no practical reason for them to be dyeing their clothing, but with plant dyes made from local barks, lichens, roots, etc. it also wouldn't have cost them anything but their time.

Every textiles culture has a variety of weaves and/or knits as well. A really simple example would be stripes or even plaids, which take more time and attention than weaving with a solid color, but not really more money. If you're a peasant stuck indoors for 3 months of winter and you have to weave something anyway, it might as well be unique if you have the skills to make it so. Plaids alone date to at least the 6th Century BCE.

There are literally hundreds of other things, from beading to cable knitting to printing and painting that people have done to their clothing for all of recorded history, there isn't enough space for me to do the topic justice really. If there's something you're specifically interested in let me know and I may be able to find out more. And just for fun, here are some really lovely color photographs of early 20th-century Ireland, with some nice costume examples.

By "clothing that we actually have photographs of", do you mean clothing that we have photographed on the original wearers (so, 1840s on) or clothing that still exists today and can be seen in photographs? Also, can you define "peasants" and "gentry"? Do you literally mean peasant folk dress, or just working class people and their clothing in general? (Because of my knowledge base, I'm going to answer right now as though you mean "1840s on" and "general working class".)

Yes and no.

People of the past really did not have the same love of individuality that we do today; they didn't feel a need to stand out visually for the sake of standing out visually. Conforming to the prevailing fashion as much as possible (without looking like you were trying to dress above your station) was key, unless you were completely impoverished. Having to do physical labor also meant that clothing needed to have few frills to get in the way, and you wouldn't want to spend much time adding fanciness to a garment that would wear out or get very dirty.

At the same time, people do like to stand out because they look especially tasteful or attractive. One's best clothes - not worn for work - would be able to have ruffles or other trim added at the maker's discretion.

That being said, to go back to the first point - these added trims were generally copied from fashion plates, and were not simply imagined out of nothing. You would want people to find you fashionable, so you would attempt to look more fashionable.