In the past, file sharing between computers took a little configuration to get done. You had to set up and properly configure your network ports, then enable file-sharing services, select files to share, and connect to an IP address for the desired system if you could not locate it with various discovery tools like the Classic Mac's Network Browser or Windows' Network Neighborhood.
These days a lot of the same configuration and setup has to be done to get the system on the network, but once on the network there are new zero-configuration protocols, such as Apple's Bonjour, which allow quick access to a variety of devices, such as available printers, shared scanners, and shared filesystems.
While convenient, zero-configuration networking in OS X does automatically search for any available shared resource and make it available for use. In most household and small-office situations this is not a problem, but on larger corporate or public networks this can sometimes result in a large number of resources being presented.
In the OS X Finder, these resources are shown in the Shared section in the Finder's sidebar, which can get cluttered with various shared computers. If your system is connected to a domain or open directory service, systems in that domain should be listed above others, but even so you may not like seeing all these available systems in the Finder.
These are just links to available systems similar to a chat buddy list showing available buddies, and as such are not there because you have a new folder or installed program on your system; however, some people might wish to keep them from showing up.
One option you have is to click the triangle to the left of the Shared section in the Finder sidebar, and thereby collapse the list of shared resources; however, this will also hide any servers you have an active connection with. Instead, use the Finder's preferences to disable showing autodiscovered network resources in the sidebar.
Go to the Finder's preferences (available in the Finder menu) and go to the Sidebar option. Here you will see a list of all the available items you can place in the various sections of the sidebar. In the Shared section, uncheck "Bonjour computers" and you should no longer see this section being populated with available Macs and PCs that have file sharing enabled. However, because the "Connected servers" option is available, you will still be able to quickly access network shares that you have established a connection with from the sidebar.
In addition to reducing sidebar clutter by disabling Bonjour computers in the Shared section, you can use these preferences to disable other unused options as well, such as custom searches and places. You can also click and drag items away from the sidebar to remove them.