ELMAH - This is the default ELMAH package with no mvc specific code in it, if you dont do what is described here I don't believe ELMAH will log any errors that occur in you're controllers, and you may not be able to access the ELMAH log page
Elmah.Contrib.Mvc - This is an enhancement on ELMAH specifically for MVC based upon one of the creators of ELMAHS answer to a stack overflow question.
Elmah.MVC - This is simliar to the above package however I believe it provides much better support for mvc features like routes etc, It is also quite easy to install and configure, it removes a lot of the messing around you would have to do with ELMAH to take out various parts that arent required when running in a mvc project
You can read the authors posts to get a better idea of how it works:
- Integrating ELMAH to ASP.NET MVC in right way
- ELMAH MVC controller released on NuGet
- ELMAH.MVC v.2.0.0 - Release Candidate
Personally I believe the last package would be the best one ot use, it seems to be the more up to date and maintained of the two mvc ones.