SourceForge is a place where open source software can be downloaded. Some truly well-made softwares, such as Azureus, Mediawiki, JBoss have been released as open source, their source code and executables hosted here to benefit everyone. There is thus some prestiege to make it here.
Imagine my surprise to find that a simple internet photo gallery was named SG, but the name is too short to be accepted by sourceforge. Thus it was renamed as Singapore. What a reversed way of naming something from its acronym!
If you want to try it out, you can go to OpenSourceCMS where various open source internet softwares written in PHP are hosted, so you can get a hands-on feel before choosing which to download and take the pains of installing yourself. Quite a nice showroom, isn't it?