Enable search and notifications for email addresses using the "+" syntax
A lot of people use a syntax such as troyhunt+foo@hotmail.com where foo is a unique identifier for the site. They do this so that if they begin getting spammed, they can identify the source their email came from.
At the moment, HIBP treats this is a totally unique email address so if I've search for the parent email address without the "+" syntax, it won't be found. This idea is to ensure that searches and notifications recognise the syntax and return addresses that are logically still the same account.
One thing HIBP would also need to do is specify which account alias was in the breach or paste. For example, I would want to know that it was troyhunt+bar@hotmail.com that was exposed in the XYZ breach.
Edit: Just to put the value of this into context, I've just run some stats on the Adobe breach. Of the the 152,989,508 rows in the dump, only 49,905 email addresses have a "+" in the address so that's 0.03% of entries. That number is also a bit high as it includes junk entries. I'm definitely not ruling this idea out - it's still planned - I just wanted to give a sense of how useful it would be.
Edit: To add to this idea, Robert's comment about a period in the email is also very valid. I'd want to be very clear about the ubiquity of this practice across mail providers, but it's certainly a good suggestion and worth further investigation.

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Anonymous commented
I have a Google apps with a unique domain, so obviously searching using the domain works, but for people using this with gmail (or similar that allows that feature) that doesn't work.
The tough part is making x+y@domain.com equal to x@domain.com but also display x+y.
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Syntax commented
I would love to have this feature, then I would implement the "+" syntax now that HIBP detect it too.
Good idea Troy :)