Is there any collection of PowerShell scripts that help the task of SQL Server maintenance?
I don't know that anyone has written something along the lines of Ola's script within PowerShell. I know that PowerShell scripts for SQL Server are slowly growing over at the TechnNet Script Center. Then MSSQLTips.com also has started publishing tips working with SQL Server and PowerShell.

Then you also have SQL Server PowerShell Extenstions project over on CodePlex. I don't guess you would consider it a repository but a good number of folks blog on how to use it and such. Some task within SQL Server are made much easier using SQLPSX.

I also tend to check out a few MVP blog sites that tend to focus on PowerShell scripts:
Allen White
Aaron Nelson
Sean McCown (MidnightDBA) (Click on the PowerShell tab for all the videos on the subject)