Welcome to the PC Gamer Bargain thread, home of PC Bargain Hunters Anonymous.
Below is some information you may find useful for searching and reporting bargains.
Several places (GOG, Humble Bundle, etc.) only list prices in US$. There are plenty of currency conversion websites around, however they often use different rates of conversion. If you are going to be paying through Paypal (often the only option that doesn't pass currency conversion onto your bank, which can incur fees), you can find out what price they will be purchasing at. Log into your account on the Paypal website, then go Profile>My Money, click on Currencies and then select Payment Conversion in the Currency Exchange box. The rest should be self-explanatory.
Viewing Other Currencies:
Most sites make it difficult to view games in different currencies, usually by redirecting (e.g. from .com to .co.uk), however some give you useful options. It should be noted however that this does not allow you to purchase in that price, and it may even be necessary for you to change it back before purchasing through the site.
Impulse/GameStop have a link under the price to switch between local and US Dollars.
Steam allow you to pick any region by appending ?cc= and a two letter country code to the url (assuming there isn't a ? already, in which case you change the ? to an & or remove everything after the ?) e.g.
USA = us
UK = uk or gb
A full list can be found here.
List of Digital Distribution Services
Compiled by Megazell on this forum.
Indie Bundle thread on the Steam forums
Where I get most of my up-to-date information about various game bundle sites.
Rock, Paper, Shotgun Bargain Bucket
A weekly round-up of deals by SavyGamer.
Useful for doing a quick search to see if there's a cheaper price around. Only searches UK retail though.
As above but works in more countries, and also searches some of the digital distribution sites.
Indie Bundle Tracker
A Steam community group for sharing information about Indie bundles, founded by doctor_roxo. Please don't cry.
If you can think of any more useful information, just let me know and I'll add it to the post. Happy bargain hunting.