I was planning to write a list of must-install-CMP(Component, Modules & Plugins) for Joomla 1.0.x and 1.5.x. However after a disastrous migration last night (I forgot to download a backup of my /images folder!), I decided to just list 1.0.x compatible first.
For new Joomla users, I reckon that its better for you to make yourselves familiar with Joomla 1.0.x first before moving to 1.5.x. The justification is simple, presently there is more components for 1.0.x available than for 1.5.x, and after some nights of testing 1.5.x, I came to conclusion that any new Joomla user who tried Joomla 1.5 straight without prior experience in Joomla 1.0.x / Mambo 4.5 will surely get deterred away. Frankly, I think this is because:
1.) they dont know yet how to differentiate 1.0.x & 1.5.x version of the particular component (present classification in JED offers not much of help),
2.) Most of 1.5.x compatible components still need to be under Legacy mode
3.) Most good & useful components are still meant exclusively for 1.0.x.
Thus, combining these three facts will result with countless error when new Joomla users tried to install any of the nearly 3000 CMP available at http://extensions.joomla.org.
Please follow the link to read the list.