SSAS Performance Monitoring Reccomendations

You can run SQL Server Profiler against SSAS instance, Extended events, and also DMVs for checking SSAS performance. Refer http://technet.microsoft.com/en-us/library/bb500198.aspx . All of the above come free with SSAS.

Spotlight and SQLSentry both monitor SSAS with different pricing.

I have used Spotlight and found it was good for general monitoring, but SQLSentry's is more detailed, but is for more advanced SSAS architects.

There are a number of different tools you can use to monitor SSAS. In general, any company that develops SQL Server tools has some kind of SSAS monitor. I didn't have a chance to work with SQL Sentry, but I know they have a tool dedicated to SSAS specifically. Spotlight by Quest has already been recommended, but keep in mind that although it offers advanced features, the higher price range needs to be considered. ApexSQL has a dedicated solution for monitoring all SQL Server services, which has a good value for the price and Idera has both free and paid solutions. Their free tool is very simple and doesn't store data, so you can only monitor the state of the SSAS in real time, and their paid tool is more complex, similar to their other products.

Just decide exactly what your needs are and I'm sure you will find an appropriate tool - the BI market is vast and growing rapidly.