It is still a alpha version of this software. So use it at your own risk.
Note: I haven't tested this software myself but many users checked it at Maemo Community.
Note 2: Registration using Yappari doesn't work anymore as Whatsapp has blocked it there is a another work around for Nitdroid users to register a account and then use it with Yappari. Trick is written on official thread and link for the blog will be after the break. and also download links will be after the break.
UPDATE 9/10/2012 Yappari 0.0.19.
UPDATE 8/2/2012 Yappari 0.0.18.
UPDATE 7/22/2012 Yappari 0.0.17.
Installation Notes: You need Qt and Qt-Mobility. All N900 should have them by default but just in case you haven't have them. here is the way to install them:
Open Xterm and type:
root (then the all of them written in quota)
apt-get install libqt4-core libqt4-dbus libqt4-xml apt-get install libqtm-contacts libqtm-systeminfo libqtm-messaging
Download Yappari v 0.0.19 from: HERE
Official Thread: HERE