Modul8 has been one of the premier VJ applications for Mac OSX since version 2.0. Following is a review based on extensive use up to the latest version, which as of this review stands at 2.5.2.
Pros

• Ability to use native OpenGL Core Image and FreeFrame plugins.
• Extensible, highly-programmable user interface.
• Excellent keyboard and MIDI mapping.
• Can use multiple video cards to span video across multiple screens.
• Good learning curve.
• Plenty of preview options.
• Frame-accurate rendering to disk.

Modul8 offers ten fully adjustable and scalable layers to composite and position in a variety of different ways. The best thing about Modul8 is that its geared towards transparency and ease of use. It wants to be instrumental and based on playing, not thinking. I enjoy mixing with Modul8, not just the end result, I dig the process of getting there, too.

TransZendent is a realtime triggering/mixing/FX software in a similar vein to Svi or Resolume. Comprising of 2 busses of up to 3 layers, with layering modes like difference, add, darken, etc and also hidden @ a keystroke. Clips are triggered from the keyboard with 27banks of 18clips, .avi, .mpg, .bmp (stills), .mov and .swf are all supported. All clip players have full speed control to 2 decimal places, negative play speeds for reversing clips, stop/play button and 2 FX slots each.