If your HTC EVO is rooted, then there's a great chance that you're running ChainsDD's Superuser, a program that grants programs permission to run with root-level access. For most people, Superuser does what it says and is more than enough to get by with. But there are times when more control and logging are needed, and developer Chainfire has released SuperSU to Google Play to address these issues.
Yes, that stands for Super Superuser, which quite frankly makes me want to slap someone, but the fact remains that SuperSU brings some functionality to the table that Superuser doesn't have and you might just want.
Features include an option to temporarily unroot so you can play movies or games that are root-restricted (Blockbuster/Google Play's movie service/etc.). It also works in recovery, which Superuser generally doesn't, and claims to work when Android is not properly booted.
There are several other features that it boasts such as "deep process detection (no more unknowns)" that there doesn't seem to be a whole lot of documentation on.
SuperSU comes in two flavors, free and pay ($2.49 USD). The pay version offers additional features such as an OTA Survival Mode (presumably an attempt to survive updates), PIN protection, and per-app logging.
Whatever you decide to do, make sure to check out the discussion and support thread (located in the description on the Play comments) before attempting to install, and for safety's sake go ahead and make a Nandroid. If you lose root for some reason, you'll want that handy to get your root access back.