Right-click on the file and choose "Open With Archive Mounter".

The ISO is mounted at .gvfs folder under your Home directory.

You can either access it from the mount location as shown above or from the side-panel in your File Manager.

Credit goes to Ubuntu Forums member, tgm4883.

There is Furius ISO Mount

for example.

1. Select the ISO you want to mount by clicking on Browse...

   

2. Click on Mount

   

3. Win

In some cases (using certain file browsers or desktop environments) the mounted image is not opened automatically, so it is useful to know the mount point

To unmount, select that line to see the mount point clearly visible beside the "Unmount" button that has now become active.

There is Gmount-iso :Click to install

Gmount-iso is a small tool written using PyGTK and Glade. It allows you to easily mount your cd images. This is a frontend to the 'mount -o loop -t iso9660 foo.iso /mountpoint' command.

acetoneiso

makes it possible to easily use various kinds of CD and DVD images on your computer as if they were burned to real CDs. You can use the application to mount and manage CD and DVD images. Supported disc-image formats are ISO, BIN, NRG, MDF and IMG. These are AcetoneISO's Features:

• Mount automatically ISO, MDF, NRG, BIN, NRG without the need to insert admin password! Only single-track images are supported for the moment.
• burn ISO/TOC/CUE to CD-R/RW optical discs
• burn ISO images to DVD-+R/RW (including DL)
• a native utility to blank your CD-RW, DVD-RW and DVD-RW discs
• A nice display which shows current images mounted and possibility to click on it to quickly reopen mounted image
• Convert 2 ISO all image types: *.bin *.mdf *.nrg *.img *.daa *.dmg *.cdi *.b5i *.bwi *.pdi and much more
• Extract images content to a folder: *.bin *.mdf *.nrg *.img *.daa *.dmg *.cdi *.b5i *.bwi *.pdi and much more
• Play a DVD Movie Image with Kaffeine / VLC / SMplayer with auto-cover download from Amazon
• Generate an ISO from a Folder or CD/DVD
• Check MD5 file of an image and/or generate it to a text file
• Calculate ShaSums of images in 128, 256 and 384 bit
• Encrypt / Decrypt an image
• Split / Merge image in X megabyte
• Compress with High Ratio an image in 7z format
• Rip a PSX cd to *.bin to make it work with epsxe/psx emulators
• Restore a lost CUE file of *.bin *.img
• Convert Mac OS *.dmg to a mountable image
• Mount an image in a specified folder from the user
• Create a database of images to manage big collections
• Extract the Boot Image file of a CD/DVD or ISO
• Backup a CD-Audio to a *.bin image (can also be burnt once made)
• Complete localization for English, Italian, French, Spanish, Polish and much more!
• Quick and simple utility to Rip a DVD to Xvid AVI
• Quick and simple utility to convert a generic video (avi, mpeg, mov, wmv, asf) to Xvid AVI
• Quick and simple utility to convert a FLV video to AVI
• Utility to download videos from Youtube and Metacafe!
• Extract Audio from a video file
• Extract a *.rar archive that has a password
• Quick and simple utility to convert any video for Sony PSP Playstation Portable
• Display History that shows all images you mount in time

Project Page

Edit: when not using Nautilus, or are using Lubuntu etc, you may not have the mounted iso readily visible: it is in fact in /home/user/virtual-drives/number. In this cases, some settings can be made to ensure the image file is opened in the file browser: go to File/Options/General options/File manager and select file browser (e.g., Thunar or Nautilus, after having installed them: in Lubuntu/LXDE I do not recommend Nautilus, but Thunar; selecting LXDE in that list seems useless).

I use cdemu, which mounts the iso as if it were an actual disc. This means that any software expectign an actual CD will work. For the GUI part, I use gcdemu, which resides in the GNOME panel. I don't know of any GUI for Unity.

Disks (gnome-disk-utilily) is a drive and partition manager that, among other capabilities, creates a context menu entry in the file manager for the given purpose.

sudo apt-get install gnome-disk-utility 

Testing in KDE, it creates a file manager panel entry called 'Loop device'. The default path to the mounted iso is /media/username/unknown.

If, after unmounting the loop device(s) from file manager the panel entries still remain there even after restarting the file manager: to remove them, run gnome-disks (the application can be searched and launched as 'Disks' too), select and close the 'Looping device', enable the "Auto-clear" option, then restart the file manager.