Recs.
Updated
SpecsUpdate
Pros
Pro Importer from many other forums
Vanilla Porter can import posts (comments), threads (discussions), users, categories, roles and passwords (hashed) from most other forum software: bbPress, Drupal, MyBB, NoteBB, phpBB, PunBB, SimpleMachines 1&2, Toast, UserVoice, vBulletin, Xenforo, YAF.NET and others. Some importers support avatars, attachments, signatures, and PMs. A few support tags and user notes.
Imports aren't flawless, but they do most of the work.
Pro Simple forum software
lightweight Internet forum package
Embeddable forums
Import from other forums
Pro Deep integration with WordPress
Use SSO with your WordPress site, adds recent discussions widgets and best of all have comments also create discussions in your forum.
Cons
Con Poor search, unless using Sphinx
The default search that comes with Vanilla produces poor results because it uses MySQL. One needs to investigate integrating with Sphinx for much better results.
Con No post edit history
Old versions of a post are only visible to admins, which may lead to a credibility problem. The amount of time during which users can edit their posts is configurable from a few minutes to indefinitely.