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.
Ranked in these QuestionsQuestion Ranking
Pro Encryption support
Pro Can overlay incoming messages
Nettalk can be set up to overlay incoming messages. Especially useful for games.
Pro Free and open source
Pro Built-in spellcheck
Pro Multi-lingual interface
Nettalk is available in English, Spanish, Dutch, Russian, Chinese, Hungarian and German.
Pro Functionality can be extended via plugins
Con German language
Is primarily a German language client, so the spell check, etc etc is in German.
Con No longer developed
Development ceased some time ago (2012)