List of useful libraries (mostly free) for common tasks, so you don’t have to write your own. Use your browser search to find the probably-useful library you need.
Apache Tomcat
[Apache]
Java Web Server, supports JSP, Servlets and Java EE.
Apache Jakarta Commons FileUpload
[Apache]
Servlet-side Library for easy management of file uploads.
Apache Jakarta Commons HttpClient
[Apache]
Make HTTP connections.
Apache Jakarta Commons Net
[Apache]
Support network protocols such as FTP, SMTP, POP3, etc.
Java3D
[JDL/JRL/BSD]
Draw and render 3D models in Java.
JavaMail
[Sun]
Send/Receive email easily. Protocol independant, but ships with IMAP, SMTP and POP3.
JFreeChart
[LGPL] [SourceForge]
Draw charts, line chart, pie chart, etc.
JNative
[LGPL] [SourceForge]
Call native DLL code using Java.