These are plain cheating modifications, which, unlike pretty much all other cheat mods, do not interfere with notch classes. Thus, they should be compatible with anything that isn't another cheat mod doing the same thing.
Simply Hax Flying:
By default press "R" to start flying, ascend with JUMP, descend with SNEAK. Also allows you to run and fly faster by holding LSHIFT.
Key bindings and max flight speed can be configured in auto-created file under /mods/SimplyHaxFlying.txt
Works in SMP with the usual restrictions.
Simply Hax Vision:
By default press "I" to make close walls disappear from your sight. Extremely useful for finding cave systems!
Key bindings and disappear range can be configured in auto-created file under /mods/SimplyHaxVision.txt
The only requirement is Forge ModLoader (FML, part of Forge or seperate).