There are lots of mods that have been made for minecraft. A lot of these mods became massively popular and used in almost anyone's minecraft.jar, such as the popular toomanyitems mod. Some mods are external programs and used for other purposes, such as the MCEdit program. There are also a ton of plugins, such as essentials and worldedit. However, there are so many tiny mods out there that are really saturating the once exciting mod market. Some mods are simply one more block added to the game, which may only serve the purpose of getting slimeballs, for example. Nice job on making a recipe or ore in the game, but there are probably 70(,000) more mods specifically oriented to easing the task of obtaining slimeballs (translation: there are many mods that make getting slimeballs easier). There are lots of mods that don't do much, or other stuff. Not to mention, like many other sections of minecraft media, such as texture packs or skins, there is usually a lot of stealing code. The mod called NotEnoughItems had to change their code so that it didn't look like toomanyitems' code, especially since they do the exact same thing, like so many mods. My point is, if you make a simple mod that just adds a single object to the game, or some mod that does something that every modder does at first, such as xp hacking, then there is probably another mod just like it, so there's not much of a point of posting it, since there are so many other mods exactly like it posted on every other site. You're just helping make the mod market less unique and fun.