Perfectly good question.
Although portable apps are not usually available under Linux,
they definitely have their place.
You can find a portable version of Opera 11.50 at :
I just found it myself and haven't yet tested it.
My use case for this type of application is primarily being able to continue
using important apps when my computer is temporarily disabled by some software
problem, which is not rare under Linux. (But had occurred too often under
the Microsoft environment before I migrated full time to Linux.)
I just need to boot with any (linux-based) recovery disk, and thus function
normally for few days it might take for my system to return to normal.
In case you're interested, there is another application called "cde" which
is supposed to let you "create" a portable app, by putting all the
requirements in a directory with the application :
Again, I haven't yet tested it.
Good luck :)