TWiki is a powerful wiki engine that seems to aim for professional use on intranets.
homepage: http://twiki.org/
Amazingly, spam does not seem to be much of an issue for TWiki. In the default install robots are only allowed to index the current version of a document (see NoIndexHistory). Another contributing factor may be the fact that the edit link simply says 'Edit' which makes it a little harder for spammers to find.
TWiki is typically set up to allow only registered users to edit articles (a LazySolution) which is ideal for its target intranet audience. This seems to be the default, and is even set like this on twiki.org. This will cut down on their spam problem (at the cost of open free useability)
However there are discussions of the spam problems and solutions on Codev/WikiSpam, and Codev/ChineseWikiSpam.
The BlackListPlugin seems to be aiming to provide an all-in-one spam solution. Presumably it is not part of the default install (a plug-in). It includes rel=nofollow delayed indexing, which could be an interesting one to try. It also includes I.P. address blacklisting/whitelisting, which is of limited effectiveness (ContentBanning is more effective, but is not included).
Twiki doesn't have EditSummaries (at least not by default) which makes it harder to spot spam.
See also AntiSpamRecommendations