Portage on Gentoo how to view available software versions?

Another way would be to list available keywords with equery, this will also show the versions available.

$ [-] equery y pidgin
Keywords for net-im/pidgin:
             |                             | u   |  
             | a a   a           p     s   | n   |  
             | l m   r h i m m   p s   p   | u s | r
             | p d a m p a 6 i p c 3   a x | s l | e
             | h 6 r 6 p 6 8 p p 6 9 s r 8 | e o | p
             | a 4 m 4 a 4 k s c 4 0 h c 6 | d t | o
-------------+-----------------------------+-----+-------
   2.10.9    | + + + o ~ + o o + + o o + + | o 0 | gentoo
[I]2.10.9-r1 | ~ + ~ o + ~ o o + ~ o o ~ + | o   | gentoo

I would've expected emerge --search to provide this functionality, alas it only seems to show latest version.

$ [-] emerge --search eix
Searching...    
[ Results for search key : eix ]
[ Applications found : 1 ]

*  app-portage/eix
      Latest version available: 0.29.3
      Latest version installed: 0.29.3
      Size of files: 536 kB
      Homepage:      http://eix.berlios.de
      Description:   Search and query ebuilds, portage incl. local settings, ext. overlays, version changes, and more
      License:       GPL-2

So to answer your question: I've been using eix for so long, I don't really remember what I did before.

$ [-] eix eix$ -c
[I] app-portage/eix
     Available versions:  0.25.5{tbz2} 0.29.3{tbz2} ~0.29.6 ~0.30.0 ~0.30.1 {clang debug +dep doc nls optimization security sqlite strong-optimization strong-security swap-remote tools zsh-completion LINGUAS="de ru"}
     Installed versions:  0.29.3{tbz2}(10:44:38 07.09.2013)(dep nls -clang -debug -doc -optimization -security -sqlite -strong-optimization -strong-security -swap-remote -tools -zsh-completion LINGUAS="-de -ru")
     Homepage:            http://eix.berlios.de
     Description:         Search and query ebuilds, portage incl. local settings, ext. overlays, version changes, and more

While I would recommend lkraav's solution, here is another way:

$ ls /usr/portage/app-portage/eix/
ChangeLog  eix-0.25.5.ebuild  eix-0.29.6.ebuild  eix-0.30.1.ebuild
Manifest   eix-0.29.3.ebuild  eix-0.30.0.ebuild  metadata.xml

this will of course only give you list of all ebuilds available for eix in the portage tree (without all the nice info which eix does provide). If you are using layman more ebuilds will be available in overlays (basically in other folders).