Dhruin
SasqWatch
We already noted Spiderweb's October sale but Xian points out Avadon is on a 24 hour deal on Steam for only $5...19 hours to go as I type.
More information.
More information.
There's a whole site devoted to that sort of thing. http://www.steamprices.com
You can compare regions and see what is fair priced (ie converts to be near the same amount) and then also the top savings (better priced in your region) and total rip-offs (worse priced in your region).