Actually there are a lot of apps with some similar features and with some differences. I'd made a short presentation for inner clients during my work to choose an app for us. As a user of BI, not a developer, I looked at app from simple point of view. I wish it works fine, do what I need for the cost we can pay. As i understood there is no full web-analog to exel which works fast with different types of cubes, has same or very close features and can be easily configured.
Your choice depends on how you want to use it, what is cost limitations and what is more important for you (work with bid ammounts of data, concrete visualisation features, easy configuration..) and what plans do you have for this app, can their support make a custom project for you etc.
Webpivotable costs not much, works fine, but their design may be better, but thats just for me. Here their demo.
icCube - nice team, support, tool totally for OLAP and their webreporting tool is enough for some usual needs, but not more. Regarding futher detailisation, this is not for you.
Flexmonster - looks nice, works good, professional team and possible different types of customization, but quite costly.
Dundas is really not bad too, actually. You need to sign in to get their demo.
If you tell me more about your needs I'd try to be more specific. These apps were the first ones I remembered.