Steam Summer Sale - hundreds of games on sale!
Steam are currently having one of their twice yearly sales. It has been going since July 11, but there is still time to snap up one of the hundreds of bargains before it ends tomorrow night (Monday) around 10pm AEST!
The last day is normally when they give further discounts on already discounted games. You can also try searching for a particular game to check the discounted pricing.
Pro tip: EB Games sell $20 and $50 USD steam cards for $20 / $50 AUD, and given the current exchange rate, this can save you around 10% ;)