ninja tabi are good boots but mobility, swiftness and sorc's are all fine options.
If the enemy team has significant magic damage, rush Bulwark. If they have a lot of physical damage then go for Locket (teamfights starting), Sunfire (slightly offensive, esp w. Liandrys) or Randuins (defensive).
If your team is doing well, you have a tanky top and you feel the need to contribute damage to fights then you can go for Liandry's.
If your team has significant magic damage, consider Abyssal Scepter.
Hope that helps give you an idea of how to build Amumu based around the champions in the game. --- ~GameFAQs LoL Board President~ http://i.imgur.com/6rO0g.png
Honestly, I just go Spirit Stone into Sunfire and Abyssal.
Then I finish the Golem Crest usually. Boots of choice since I've already got armor and Mres.
Then I get Liandry's but I'm sure there are better things to do than this. Like most of the other stuff in here I suppose. --- http://img545.imageshack.us/img545/6840/mathz.jpg http://i.imgur.com/iB1sU.jpg http://i13.photobucket.com/albums/a290/juliaxx/Picture56.jpg Free stuff!