In my years as a web designer I can't remember how many times I've had to theme a back-end admin system for one reason or another. It seems to be a really common task so when we started ThemeForest a couple months ago I added an "Admin Skins" category for back-end admin themes and it's turned out to be the most popular category on the whole site. It makes sense because a client admin area often just needs to look neat, tidy and professional, so spending ten or fifteen dollars on an HTML skin is a pretty economical way to make the job look nice and slick, especially if design isn't your primary skill set. The themes generally have a range of styles for things like tables, lists and even calendars, so they are pretty flexible and can be moulded to fit most little applications.
Here are 7 of the best admin skins, note that because they are via ThemeForest, they are paid themes. But hey in my extremely biased opinion they are worth it :-)
1. eManager 1.0
Features: Three level menu (nav, subnav, tabs), stylish form fields and table, and a neat web 2.0 look.
Features: Tons of navigation options, calendar, jQuery drop downs, and clean, crisp styling.
Features: Three colour schemes, jQuery drop downs, sharp looking layout.
4. Admin Nano
Features: Four colour schemes, calendar, two level nav, alert/error messages.
Features: Super flexible layout, three colour schemes, whole bunch of themed pages.
Features: Simple, clean, 2-column layout
7. Ninja Admin
Features: Super simple, clean layout.
And you can find plenty more themes like these in the Admin Skins category on ThemeForest.