NettalkOpen source (licensed under Apache 2.0)
-Supports xdcc
-- with filesizes over 4GB (like mirc and xchat(and xchat too tends to fail here if combined with resume...))
-- supports spaces in filenames (unlike KVirc for example)(with the caveat that it replaces the blanks with underlines in the downloads)
--will fail with some special characters in filenames (like Commie's
Fate ⁄ Zero). Otherwise the client is utf-8 enabled (chans, nicks and text)
Caveats;
-- doesn't support IPv6
-- doesn't support SSL
-- German client and only half of the documentation has been translated so far (see link)
From personal experience, if left running for very long, or too many open channels/active downloads, it tends to crash sometimes...
Despite the caveats mentioned, it's pretty decent client for xdcc purposes...
For the rest see the link above