The mark of stars feat (Dragonmarked, p142) prevents you from being flat-footed no matter what. Unfortunately, it requires taking a level in heir of Siberys (Eberron Campaign Setting, p80).
The adventurous explorer feat (Dragon 315, p64) gives the following benefit: "When you ready an action in combat, you can choose to take that action at any time, not just when the condition you specify occurs." That's kind of like an immediate action.
The mark of Avernus feat (Fiendish Codex II, p84) lets you attack or use a spell-like ability as an immediate action once per encounter. Unfortunately, it mostly requires being a devil.
The divine defiance feat (Fiendish Codex II, p83) lets you counterspell as an immediate action by spending a turn undead attempt. It's way better than reactive counterspell, which has that "takes the place of your next turn" clause.