Judging by your enjoyment of Insomnium, I gather you'll like the following varied acts:

In Flames - Swedish band. Mainly their older work. Check out some stuff off of 'The Jester Race' and 'Colony' albums to get a taste of their work. They've got generally a faster, aggressive sound. Moonshield, December Flower, Colony, and Zombie Inc. are great songs to start with. December Flower in particular has a killer solo right in the middle.

Novembre - Italian guys - a rarity for the genre. You'll like the 'Classica' album. Harsh vocals but with an expressive melancholy sound. Some really good guitar melodies here. 'My Starving Bambina' and 'Winter 1941' are great tracks.

Opeth - Another Swedish group; probably THE band of the genre, if not underground metal as a whole. They've hit it huge recently and are completely deserving because... well, they are fantastic. I think 'Still Life' and 'Blackwater Park' are their best releases, but you'll find a lot of opinions about which is really their best work. Try the songs Bleak, Moonlapse Vertigo, Harvest, Face of Melinda, and Godhead's Lament.

There are tons of other bands out there, but I figure these are good starting places. Hope you enjoy.