Yak!
A serverless, text-based, chat application for use on Microsoft Windows LANs. It supports: A simple and easy to use interface, customized playback of sounds on message receipt, user grouping, URL/email highlighting, file transfer, encryption of messages, smilies, user blocking, various...
http://www.digicraft.com.au/yak/YakSetup.exe - 1Mb - Download - Screenshot

SnapTalk
SnapTalk is the cross-platform private chat and file transfer utility for your office. Instantly send messages and files on your local network. More powerful than standard chat, SnapTalk includes full featured archiving and search of transcripts. In addition, SnapTalk includes customizable... SnapTalk uses peer-to-peer technology and unlike public chat systems, does not rely on Internet servers. ... You can be sure that everything in SnapTalk is company business. ... SnapTalk seamlessly connects Mac and Windows computers!
http://www.glassbead.com/download/files/ST_WinInstaller.exe - 10Mb - Download - Screenshot

Absolut Chess
Absolut Chess (AC) is a funny and complete chess for all players. It allows battles in computers nets (LANs and Internet) where your opponent plays remotely with you, or front-to-front battles, where the two players combat in a single computer. When the choice is to face the machine, AC presents...
http://www.overlans.com/ACSetup.exe - 2Mb - Download - Screenshot