I explored some Alpha and Beta 1.7.3 Minecraft servers that are still up giving a tour around them and looking at all the cool builds. Most of them have little to no players left which was sad to see although the last two had some more activity. If you think any of them are cool I recommend going to visit them for yourself!

Server IP addresses:
Alphaplace.net (beta 1.1_2)
Alphaanarchy.net (beta 1.2)
Alwaysalpha.net (alpha 1.2.6)
Oldschoolminecraft.net (beta 1.7.3)
mc.retromc.org (beta 1.7.3)