This one is fast and easy to use. Now, Windows user and Mac user can chat easily with this freeware. This one is English version and you have to Japanesize it. You need to download this patch to make ICQ for Mac 2.0 Japanese capable. It will let you chat and send instant messages.
Japanese page is here.
Talksender can let you exchange voice and pictures. You don't need any additional software to record your voice. This one will import most of the address book from your e-mailers. The best thing is that it goes across the platform. This one is fast and easy to use.