If you use GNOME 3 and Adwaita theme, GTK2 applications look different (as in ugly) than GTK3 apps. That's why maximo1010 @ Ubuntuforums has ported Adwaita GTK3 theme to GTK2.
The theme is called Advaicium and is based on the original Clearlooks. The theme is not perfect but considering the different engines GTK3 and GTK2 themes use, I'd say it looks really good.
Using Advaicium, you will basically continue to use Adwaita for GTK3 applications but GTK2 applications will use Advaicium.
Using Advaicium, you will basically continue to use Adwaita for GTK3 applications but GTK2 applications will use Advaicium.
Here's a screenshot with two GTK2 applications (Shutter and Transmission) using the original Adwaita GTK3 and Advaicium in Fedora 15 (for comparison, I've also added Nautilus, a GTK3 app):
To install Advaicium in GNOME 3, extract the theme, copy the Advaicium folder to /usr/share/themes/ and use GNOME Tweak Tool to change the theme to Advaicium.
To install Advaicium in GNOME 2 (I guess there aren't many people that want to use it in GNOME 2 but anyway), copy the theme to /usr/share/themes/, open the Appearance Preferences, select "Adwaita", click "Customize" and under the "Controls" tab, select "Advaicium".
Please note that Advaicium is still in alpha so you may find bugs!
To install Advaicium in GNOME 2 (I guess there aren't many people that want to use it in GNOME 2 but anyway), copy the theme to /usr/share/themes/, open the Appearance Preferences, select "Adwaita", click "Customize" and under the "Controls" tab, select "Advaicium".
Please note that Advaicium is still in alpha so you may find bugs!
Download Advaicium (Requires an Ubuntuforums account)