Following the release of Aperture Desk Job - Valve's free playable "short" set in the set in the Portal universe - a spot of eager datamining has uncovered references to four unannounced Valve games, including a Half-Life: Alyx follow-up, a Source 2 port of Counter-Strike: Global Offensive, a recreation of a cancelled Portal title, and Citadel, a Half-Life shooter/RTS hybrid.
All this was uncovered by long-time Valve watcher Tyler McVicker, with at least some of the new discoveries pointing to titles long rumoured to be in development at Valve, but never actually confirmed. The caveat, of course, is that their mere existence as references in Aperture Desk Job's code doesn't mean these are actual games still in development (particularly given Valve's well-reported experiment-then-ditch approach to game design), or that they'll ever see that light of day. As McVicker says in a video documenting his discoveries, "take everything that we're about to discuss with a grain of salt".