Nettalk is a free and open source IRC Client for Windows. It is a fully-fledged IRC client with UTF-8 and DCC file transfer support.
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Pro Can overlay incoming messages
Nettalk can be set up to overlay incoming messages. Especially useful for games.
Pro Encryption support
Pro Multi-lingual interface
Nettalk is available in English, Spanish, Dutch, Russian, Chinese, Hungarian and German.
Pro Built-in spellcheck
Pro Functionality can be extended via plugins
Pro Free and open source
Con No longer developed
Development ceased some time ago (2012)
Con German language
Is primarily a German language client, so the spell check, etc etc is in German.