WBB-Elite offers free and premium plugins for WoltLab Suite, managed hosting and managed vServers.
cls-design is the largest manufacturer of premium products for WoltLab Suite™. They offer a variety of high-quality styles, apps and plugins.
SoftCreatR.dev is a team of experienced web developers, who develop applications and extensions for WoltLab Suite™ to enhance your community.
Elevenfour offers free styles & plugins for WoltLab Suite™. Furthermore there are some tutorials around web development - both articles and videos.