My 3 year old collie cross is lucky to be alive!  Everytime we go out for a walk, and I take her off her lead, she goes absolutely crazy at being off the lead, and if there is another dog around, she chases up to it, barking with her heckles up.  If the dog tries to run, she chases and will nip if she gets close enough.  If it stands its ground, she will come away, still barking, but when it moves again, she will chage up to it again.  She does come back when I call her, sometimes not straight away.  I have another dog, 5 years old, she used to, and sometimes still does, bark at bigger dogs than she is, maybe the younger one has got a bad habit off her, but its much more worse.  Does anyone think this is nervous aggression or is she just a raving lunatic.
Thanks,
Josie39

My initial advice would be to keep her on the lead, at least until you are able to sort out the problem.  Consideration for other owners and their dogs must be the main priority.

This sounds very much like under-socialisation and poor training coupled with the collie instinct for controlling anything that moves. :( I'd suggest some good training classes, and not let her off the lead where she's likely to bully other dogs and possibly cause a fight. Also walk your two dogs separately so they don't start 'packing up'.