Virtual Machines
Look at the Status Page for info on the current platforms.
Pro's:A direct port of the Sun VM's. Consequently compatibility is very high. JPDA Support in 1.2 version. 1.2 & 1.3 VM supports Java3D, JMF and JavaAI.
Con's:Not directly supported by Sun. Not open source.
Pros: Similar to Blackdown VM, but with Sun seal of approval. Released at the same time as Windows & Solaris VM's.
Cons: Releases don't come out as often as Blackdown. Not open source.
Pros: Rediculously fast particularly for CPU intensive loads. Passes Sun's compatibility tests.
Cons: Not open source.
Pros: Has a JIT for JDK 1.1.x. Open source. Runs on tons of platforms.
Cons: Not as compatible as non-open source solutions. No JDK 1.2, 1.3, or 1.4 support.
Pros: Open sourced, with LGPL. Conforms to Mozzilla OJI spec.
Cons: Seems to be least far along of all projects. Performs poorly (no JIT).
Pros: High quality, native code compilation for maximum performance. Server only (no GUI), no support for AWT, Swing or WFC.
Cons: Not certified as Java compatible (although appears to have excellent compatibility). You can have any two of cheap, fast, and good quality. ;-) Not open source.