The Sitecore IndexViewer is a Sitecore application which lets you browse and search the Lucene indexes in Sitecore. It has now been released for Sitecore 6.1.
Sitecore 6.1 included a fix, which makes the Lucene indexes a lot more stable. They also upgraded Lucene to version 126.96.36.199, which meant that the IndexViewer didn't work in the new version.
The IndexViewer has now been upgraded to match the API changes. You can download the new version here: http://trac.sitecore.net/IndexViewer