For a mere dollar you can get the Dreamcast Collection, NiGHTS into Dreams, and Sonic & All-Stars Racing Transformed for PC on Steam!
The Dreamcast Collection includes Sonic Adventure DX, along with Crazy Taxi, Space Channel 5: Part 2, and Sega Bass Fishing. Those ports are kinda crappy but Sega has recently been working to update and fix them up a bit.
NiGHTS is solid though. And Racing Transformed is pretty much one of the best kart racers ever.
More games will be unlocked for the beat-the-average tier next week. Maybe more Sonic games will be included? Maybe. Either way, you could buy the dollar tier now and upgrade later. (The average price to beat will be locked to whatever it was when you bought the bundle. Typically it only goes up over time, but not by too much.)