Options
Both Thunar and PCManFM are valid alternatives to Nautilus and more responsive in general, with PCManFM being ever so slightly faster than Thunar. As both of these options have already been mentioned in different answers I would like to point you to yet another alternate file browser: Dolphin.
Dolphin
Out of all file managers on Ubuntu, Dolphin it is both the fastest and most feature-rich. However, as a Kubuntu application it comes with a long list of KDE dependencies. These won't harm you but will take up additional disk space. And because Dolphin does not use the shared GNOME libraries it will likely take slightly longer to start than the aforementioned alternatives.
Installation
You can install dolphin with:
sudo apt-get update && apt-get install dolphin --no-install-recommends
Setting up the default
Here is a Q&A on setting an alternate file manager up as Ubuntu's default:
How to set up Dolphin as default file manager?
This Q&A deals with Dolphin, specifically:
How to run Dolphin instead of Nautilus?
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or something while you're using it? – Ash Mar 31 '13 at 12:44