There is no single best/most popular Latvian forum as far as I know.
But there is a lot of user generated content on our national social networking site draugiem.lv (internationally known as frype.com). There are groups (like usenet newsgroups but web 2.0) about every topic imaginable. They are like forums except you have to become a member of the group before you can read/participate in discussions (which is pointless because the most popular groups will approve you anyway). There is a classic forum on draugiem.lv as well.http://www.draugiem.lv/groups/http://www.draugiem.lv/forum/
Auto http://www.audi-style.lv/forum/ http://www.bmwpower.lv/forum.php http://www.iauto.lv/forum
IT/tech http://www.boot.lv/forums/ http://eoz.lv/ http://datuve.lv/forums/ http://php.lv/f/
Women http://cosmo.lv/forum http://www.kleoo.lv/forum.php
A word of caution
1) Latvian forum is not the friendliest place in the world
2) While Latvians generally can spell, the vast majority chooses to ignore punctuation
Cosmopolitan is probably the friendliest Latvian forum and kurbijkurne the most intelligent one.
You can also find lots of other forums by doing a simple Google search.