The vast majority of Linux software is free.
RPMFind is not a good place to find software, it is mainly useful for finding downloads for your version of Linux when you know what software you are after.
Firstly, have a look on your system, Mandrake ships with programs to do just about everything.
Second port of call -
http://www.kde-apps.org - Lots of handy software.
Third port of call - Search on
http://www.sf.net sourceforge (Searching the site with Google might be better).
Unlike Windows there are a huge variety of apps for just about everything. I would recommend:
Desktop Manager - KDE 3.3 (This should already be installed and running on Mandrake)
Office Suite (Word Processing etc.) - Get the OpenOffice.org 2.0 Beta from
http://www.openoffice.org
Music Player - Amarok
Web Browser - Mozilla Firefox
Email - Either use web-based email or Mozilla Thunderbird
Image Editor - GIMP 2
MSN Messenger Equivilent - Kopete
Development - KDevelop (if you want to use C++)
BlueGUI Windows Plugin