Mind you - it's also like comparing apples to oranges also, because ultimately, they are totally different bands of totally different styles. Reastically, you can't compare them.
I think you could count Sinergy as a side project, Alexi Laiho and Roope Latvala both played in it, but Roope wasn't in Children Of Bodom at the time. For Alexi it was a side project for sure.
Ahnenstahl - solo side project from the main man in Rivendell
Raventhrone - Pazuzus other project
Blackfield - Steven Wilsons side project besides Porcupine Tree
Ruins of Beverast/Graupel/Verdunkeln - All projects springing from the death of superb Nagelfar
Forest Silence - Sear Bliss Side project
Havayoth - Prog side project from Vintersorg
Nachtmar - Black metal side project from the founders of Empyrium
Mistigo Varggoth Darkestra - Solo project from the main man in Nokturnal Mortum
October Tide - Katatonia side project
Odroerir - Project from some main members in menhir
Ice Ages - Summoning side project