Here is a good paper that I found online that tells you the how, why Phasers work and the differences in some of the better known ones like the Univibe, Phase90, Phase45, etc.
I just wish that he could break down the "sound" differences of each rather than talk about all the tech talk.
For those of you VH freaks out there like me - This is good stuff for Phase90:
diy.erikhansen.net/phase90.htm
(just paste it in the address bar like that - no www.)
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"When we went on our first tour and were opening for bands, the other guitarists used to laugh their ass off when they saw my pedal board......They stopped laughing when they heard me play."
- Ed on his homemade Plywood Pedal Board