Yahoo Instant Messenger
I recently tried installing the Yahoo Instant Messenger 9 but found it had lots of problems connecting to the network. I reverted back to version 7 and found that I'm still getting the same problem, but after several tries it finally connects. I actually do not use standalone versions, I use Miranda instead, but recently needed voice and I'm getting all these "Cannot connect with the network" errors.
I actually despise any kind of messenger system, but with Yahoo it is necessary for international calls as I pay just a few cents. They are also necessary for Discussing business with clients, but other than that I keep them offline. Interestingly enough they say that it is best to upgrade to version 9 in order to avoid firewall problems, but it was quite the opposite effect for me.