Miranda's contact list is implemented in plugins. Only one contact list plugin may be active at a time. By default only the classic contact list plugin is installed. If you want to try out the other contact list plugins mentioned here you should go to the Miranda IM download page and download the Extra Plugins Pack.
Classic contact list
"Classic contact list" (aka clist_classic) is the default contact list. It looks and feels like a native Windows application, and is straightforward and fast. It does not provide skinning facilities, although fonts, background colour, and a background image may be set within Options->Contact List.
MultiWindow Contact List
"MultiWindow Contact List" (aka clist_mw) is a branch of Classic contact list that supports multiwindow frames and Metacontacts. Frames allow plugins to add little windows to your contact list. It is possible to move frames around, dock them inside your contact list window, or keep them out on your desktop. Such plugins include world clocks, ping status monitors, toolbars, media player controllers and system statistics viewers. clist_mw also has the option to show contacts' status message below their name on the contact list, and extra icons to show extra info, such as the client your contact is using. clist_mw still does not have any more to add to Classic contact list's customisation options.
Modern Contact List
"Modern Contact List" is a truly skinnable contact list. Naturally not as small as the first 2, but offers extremely powerful skinning capabilities. It is descended from "Meta MultiWindow Contact List" (no longer developed mod of MultiWindow Contact List), and hence includes basic features from that. Skins can be downloaded from the File listing, and they are varied and plentiful. clist_modern has built in support for Metacontacts integration, frames, and transparency. clist_modern supports Windows 9x and above (even basic transparency on 9x).
Thanks to tweety, you can find a page of skins and previews here.
"CList Nicer+" is the maintained version of Clist Nicer, descended from Classic contact list, and was the first attempt to make Miranda's contact list 'nicer'. It adds various transparency options (TweakUI needed), and rounded window corners. CList Nicer+ supports skins, which are also available on the File listing. This plugin is considered to be complete and new features won't get implemented. However, compatibility with Miranda will be maintained and bugs will get fixed.