RapidShare is a popular choice to host files online for free. RapidShare has generous limits (200MB file upload limit) even in a free account. To download a file, simply use the link for the file. If you want to share files with friends, upload the file to RapidShare and send the links to download.
RapidShare can be really useful if you know how to get the right files. On RapidShare, you can get every type of content you could possibly dream of - warez, songs, pirated content, pr0n etc. One feature they miss (and I wonder why) is searching for files. You could use complicated Google search operators, but to make your job easier, here are the best search engines to search for content you need, on RapidShare. Simply type your search terms and hit enter - just like Google.
Among the above, SearchShared can be used to search on many other sites (simply tick the ones you want). RapidShareLink has the best design. ShareMiner has a Firefox plugin, so if you use Firefox, you can search for Rapidshare files from within Firefox.