This addon can replace smileys in mIRC windows with corresponding images (a-la trillian), perform regexp highlighting (should work for case-insensitive highlighting on non-english locales), and use random quit messages on exit (a-la bitchx).
Changelog: v0.27s: mIRC 6.34 supported, v0.26s: source code added, v0.26: bugfixes, v0.25: added "graphical interface for configuration" and transparent smileys.