I’ve covered this one before, but it is so awesome it needs to be mentioned twice. It really is that good. What does it do? It latches your vCenter, and will check your clusters for HA/vMotion compatibility, and will let you know what is broken if there is something ‘off’. They have several videos and downloads available from their site here.
Have another tool you’ve found useful? One that you wish someone let you know about before you started virtualizing? Let us know in the comments.