Please answer the following questions - they will really help us identify what resources you are using, and it will probably help you better understand your own request:
1) How are the users going to upload to your server? (what software are they going to use, what operating system are they going to use)
2) How are the users going to download from your server? (what software are they going to use, what operating system are they going to use)
3) What software are you running on the server that they will upload to? (what server software, what operating system, is java available)
4) When you say upload and download... what do you mean? (ftp, web page uploading/downloading, p2p, generic file transfer over TCPIP)
As you can see... there are a lot of things that you need to tell us first. Based on (some) of the answers to the questions above, I bet someone will be able to provide some links or suggestions.
Cheers,
Jeff