This game from Sigma Team is an easy to pick isometric shooter.
Despite it's age and graphics, the fun factor is great.
Also, the small but effective improvements you can buy allow you a bit of customisation
on the gameplay style.
Different weapons ARE different, and you have to be careful with reload times.
Hordes and hordes of aliens come to get you. You can also use the environment on your
favor, like doors , corridors ans such.
Sound assests are good. When you mix aliens with explosions, grenade launchers and mayhem,
it gets really really rewarding.
Usually this game along with others (from Sigma Team) appear on bundles,
or heavily discounted not only on steam but also in other sites (i.e. Indie Gala).
When you are bored, and want a quick and mindless session you can fire a chapter
, or even play a survival session.
Play it with v-sync off, to avoid some problems.
It runs on a toaster, so it's perfect for those (like me) that have ancient hardware around. http://steamcommunity.com/sharedfiles/filedetails/?id=302142791