Cadenza Interactive is offering a free weekend of Retrovirus and Sol Survivor on Steam.
Retrovirus is a pretty fantastic game. It's a six degrees of freedom (6DOF) shooter in which you play a heavily-armed and highly mobile Agent of a PC security suite - essentially the wetworks component of Norton Anti-Virus. The PC you inhabit has been infected with a highly destructive virus, and it's up to you to clean it out. There's a bit of a Tron 2.0 aesthetic to it but the gameplay is very much in the vein of Descent, as you float freely through tunnels and chambers, eradicating the viral infection as you go. The single-player mode is big and intense, and once that's taken care of there are competitive and cooperative multiplayer modes to jump into as well.
(I haven't played the turret defense game Sol Survivor but apparently it's quite good too. 75 on Metacritic!)
Why am I telling you this? Because both games are doing a free weekend on Steam, meaning that you can grab them for precisely zero dollars and play your face off from now until October 7. And if it turns out that you like them, it's also a good opportunity buy them: For the duration of the free weekend, both games are on sale for 60 percent off. But regardless of whether or not you end up opening your wallet, it's a great opportunity to try something different (and really good) over the weekend.