Coolplayer is a fast open-source audio player. It has a simple user interface and has a lots of skin support.

Miranda IM is an open-source multi-protocol instant messenger client for windows. It's smaller, faster and easier!
For more details on Miranda click here

Download:-
Miranda v0.8.5 for Windows
size 1.63mb

Sumatra PDF is a slim, free, open-source PDF viewer for Windows. Portable out of the box. Sumatra has a minimalistic design. Simplicity has a higher priority than a lot of features.
It's small and starts up very fast.

It's designed for portable use: it's just one file with no external dependencies so you can run it from external USB drive.

Bigzip is open-source compression utility tool based on Java.

7-zip is an open-source file archiver with high compression ratio with support for 7z, zip, gzip, tar files etc. It has been found that the compression ratio of the new 7-zip in the 7z format is lot better than the best known compression softwares like Winzip and WinRar!!

Features of 7-zip

Download:-
size 1mb
7-zip for windows

This is an awesome P2P file sharing software, probably the fastest one in its category and the best feature is its open-source!!

Using Sababa DC you can exchange any files over direct connection protocol. This software is a client free of spyware. Using Sababa DC you get the power over DC networks. You have the right to create your own servers and communities. Check out its features.

Download:-
size 4.4mb

Sababa DC 1.02 for Windows

Google talk is a chat client for the gmail server. Google talk is now Open-source.

Pidgin is an easy to use free chat client used by millions. You can connect to any multiple chat networks using this client i.e googletalk, yahoo, AIM could be connected simultaneously!