Of course Ubuntu does come with an archive manager of its own (the one that comes with Gnome actually) but when comparing with 7zip... I gotta say.. that archive utility looks stupid. By default 7zip supports file formats such as...
*. 7z, XZ, BZIP2, GZIP, TAR, ZIP, WIM, ARJ, CAB, CHM, CPIO, CramFS, DEB, DMG, FAT, HFS, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, RAR, RPM, SquashFS, UDF, VHD, WIM, XAR, Z... copied from its official page.
*. Add passwords.
*. Change compression levels.
*. Save in various formats.
*. Split-merge archives.
*. Encryption manager.
*. Has a file manager of its own!.
*. Create self extracting archives.
*. Lot of language support.. are very few to mention.
If you want to install 7zip in Ubuntu 11.04 Natty Narwhal, then the easiest method is to use the "Software Center". To do that, open the Software Center and enter the "7zip" into search box and follow instructions on the screen. Good luck.