Opera is one of the bests , and you can download it for your Linux Distro from its website
But if you would like to be able to upgrade your opera using apt with your Debian Lenny you better use repositories.
You have to edit your /etc/apt/sources.list, and add this line.
deb http://deb.opera.com/opera lenny non-free
Now you need to import the gpg-key.
gpg --keyserver subkeys.pgp.net --recv-key 6A423791
gpg --fingerprint 6A423791
gpg --armor --export 6A423791| apt-key add -
and the update de database
Now you have your source installed and your database updated you can install opera.
sudo apt-get install opera