Enkitec's Session Snapper for Exadata - or ExaSnapper - is an Exadata-specific performance troubleshooting tool. ExaSnapper is able to "peek" inside what's happening in a storage cell when it is executing your queries, Smart Scans or just plain cell IO. As it uses only the "cell" metrics from V$SESSTAT and a few other V$ views, any developer or DBA who can log in to the database, can use it. There is no need to log in to storage cells or run CELLCLI commands. ExaSnapper is meant to complement the ASH data and SQL Monitoring tools, not replace them. You would usually need ExaSnapper for advanced troubleshooting of Exadata storage cell specific problems, where traditional diagnostics approaches with wait events and SQL Monitoring reports are not detailed and deep enough.
You can watch a series of videos demonstrating ExaSnapper here: http://enkitec.tv/2013/02/22/tanel-poders-exadata-snapper-hacking-session-videos/
Note: ExaSnapper is still in beta and will likely undergo major changes by the time it reaches version 1.0