Are there any alternatives to Nautilus?

Thunar File Manager is the Xfce file manager. It is quite clean and should provide the functionality you are looking for.

Other than that I only know PCMan File Manager which should be quite similar to Thunar.

PCMan File Manager. It is more responsive than thunar and nautilus. The interface has more real estate.

No one has yet mentioned the excellent qtfm, which, for a lightweight filemanager has some very useful features. It is based on 'pure QT' and does not bring with it a huge amount of dependencies like other filemanagers might. It can be installed with the Ubuntu deb or compiled from source. Please see the readme for further general information about the filemanager and how to compile it.

As noted on the official site, it has such features as

• full theme and mime filetype icon integration
• tree, bookmarks, list, icon, detail and thumbnail views
• customizable interface, rearrange views and toolbars to suit
• customizable key bindings for built-in and custom actions

Importantly, you can also configure custom actions by going to edit > custom actions and edit or add some new ones, very much in the manner of Thunar's custom actions. In the second screenshot below, the default custom actions are listed and is quite easy to follow the model and add your own actions.

You can set mime types by going to edit > edit filetypes, and set multiple shortcuts by going to edit > configure shortcuts, so the filemanager seems very configurable.

The performance is very good and you can increase that even more by running it in daemon mode with qtfm -d &.

You can manually enter file paths in the location bar, which you listed as an important feature for you in your question.

As you can see in the first screenshot, the default layout is simple and very usable, and you can have icon or list view and folders can be opened in tabs. The tabs feature may be particularly useful if you need to flick between folders quickly.

a) The main filemanager window:

b) Custom actions dialogue:

(You should still keep Nautilus installed, for example, as it will still be necessary for volume management and for drawing the desktop.)

Just to add a few options, 5 easy to install alternatives (Dolphin, Thunar, GNOME Commander & Nautilus-Elementary): http://www.omgubuntu.co.uk/2010/06/daily-5-5-alternative-file-managers

Also Marlin: http://maketecheasier.com/using-marlin-file-manager-as-nautilus-alternative/2012/02/02

I would recommend Nemo. It's based on Nautilus, but with a few handy improvements (such as typeahead). For a full list of improvements, see Here. Please note, although Nemo was written for, and is dependent on the Cinnamon desktop (which might cause Unity some problems), there is one with Unity patches available. You can install it using these three commands:

sudo add-apt-repository ppa:webupd8team/nemo
sudo apt-get update
sudo apt-get install nemo nemo-fileroller