Blockland ships with a few fonts, such as Palatino Linotype (default for chat), Arial (guis), Arial Bold, Impact, and Verdana. The engine is able to load in any font the client has available on their computer, however different OS's have different default fonts, so you'd need to look up what's available.
However, some users have their font cache set as read-only, meaning the engine can't load in fonts. So the only truly safe way is to stick with the pre-loaded fonts in a few select sizes.
Any font and size found in base/client/ui/cache on a clean installation is safe to use.
