I am signed up to get Southwest Airline's discount emails. Once they had 40% off all flights and I do see that they frequently have good discounts, deals, etc. Do other airlines such as United, Virgin, Jetblue, Delta, Alaska, etc have such discounts?
If you sign up for the JetBlue emails, you should receive discount emails on most Tuesdays (the promotion is usually for one-way fares for $49, mostly for travel in the vicinity of the West Coast). This seems to be a perpetual offering from JetBlue, not a one-time promotion. I am a seasoned traveler (flying at least several times a month), and, to my knowledge, the more conventional airlines (Delta and United among them) do NOT offer such discounts. Virgin tends to have monthly promotions, though I'm not sure you can receive email updates. Check their homepage for these offers. The same goes for Frontier, which offers amazing deals on its website. Finally, while Alaska doesn't seem to advertise its promotions via email, they consistently (again, not occasionally) have remarkably reasonable fares (especially between the West Coast and Hawaii). Happy traveling!
I've found that FareCompare Alerts do a reasonably good job of tracking all airline deals in one place. You can put in your home airport and set up either instant, daily or weekly emails to notify you when prices drop or when new deals are out. They have the usual price data but there are also real people on their team who (I assume) subscribe to airline newsletters and put those fares in manually.