Who doesn't love a pretty terminal? The prettiest I've found is by compiling iTerm with black tabs and Solarised Dark. As for dotfiles, it's hard to look past YADR although if you're pretty happy with your dotfiles, I'd still recommend checking out FASD.
As for customisation on OSX / Unix, UnixPorn is brilliant. Especially for posts such as an experiment by Knubie who wrote an excelent gist on getting something like this together.
In the world of IRC, Weechat is far superior to Irssi as a terminal based client.
Supporting C, Python, Perl, Ruby, Lua, Tcl and Scheme - it's very extensible and open source. I personally recommend it to anyone looking to move away from Irrsi.
For Windows, you can always run it in Cgywin, but maybe you're intersested in getting the most from the standard windows command-line. A project inspired by Homebrew for mac might be able to help with this called Scoop. Comparable to Ninite, it'll definitely cut down on the times you're aimlessly clicking 'Next' on installers.
While we're on the topic, you might also enjoy Beets, a terminal based media library management system. Handy for automating a ton of really annoying work which commonly occurs with a music library.