In no particular order, because I don't know which were profitable or which represented a larger portion of revenue:
- Subscriptions (users could pay for a few extra features and to disable ads on the site) - DeviantArt branded merch. - Prints and products with users' art printed on them - Sponsored Contests. These promoted movies or other media properties, or software of interest to artists. Often the prizes included Wacom tablets and Adobe Photoshop licenses.
During my time there a significant problem we were dealing with was due to deviantArt's stance on adult content. Anything was allowed as long as it wasn't outright pornography. In practice that meant that nudes were allowed but sexual acts were not. This had consequences for deviantArt's revenue. It meant that we could not run ads from the "reputable" ad networks and were forced to deal with seedier outfits that often (e.g. constantly) included malware in the display ads, exposing users to all sorts of nasty stuff. One of my projects was to detect and/or prevent the malware ads which proved challenging and at least given the amount of resources devoted to it, it was not very fruitful.
It really is sad for me to see what deviantArt has devolved into. Once the original founders sold out a few years ago I really didn't hold out much hope for the site's future.