A feature that distinguished Unreal was the ability to pick up and store useful gadgets for usage at another time. This overview sheds some light on these inventory items, as well as the more classic health pickups one can collect on the way.
Collect some Bandages and you'll gain +5 health, up to your maximum of 100 health. Pitiful, you may think, but hey it's still health you wouldn't otherwise have, and you often find Bandages in small clusters thus giving you more than +5.
Collect a Health Pack and you'll gain +20 health, up to your maximum of 100 health. Always useful to have around, Health Packs are things you will often go through phases of having too many of and not having enough of. But of course, you'll just have to remember where you left your surplus and come back for them later.
Nali Healing Fruit
Eat a fully-grown Nali Healing Fruit and you'll usually gain +29 health towards your maximum of 100. Occasionally a fully grown Nali Healing Fruit will give you slightly more or less, and of course a Nali Healing Fruit that isn't yet fully grown will give you much less. Seen normally outside but also withing old Nali buildings and underwater, these fruit have the ability to grow pretty much anywhere, and are a useful health boost.
Super Health Pack
Super Health Packs will give you +100 health on top of whatever you've got up to a maximum of 200. This raised maximum is what gives Super Health Packs their key advantage, but you should still take care of yourself because you'd be surprised how quickly 100 extra health points go down the tube. Due to their power, Super Health Packs often take some work or sacrifice to get to. Be prepared to explore.
Finding a Universal Translator is essential to your mission. It will decode countless messages that give you clues about your surroundings or hints as to where to get some powerful stuff. It never runs out, and provides you with information throughout the game. Luckily, the game pretty much forces you to collect it. Note: there is a shortcut to using this as opposed to selecting it from the inventory and activating it. Press F2.
Flares are a useful source of temporary light. When you are in the dark and need more light, simply toss one or more of these (you can carry several of these at a time) to light up the area, even underwater. Flares have the advantage over Flashlights and Searchlights that once a Flare is dropped it provides a general glow that lights the area, rather than a highly focused beam of light. That is, until it burns out. Most of the time there is plenty of light, but sometimes there's just a dark corner, or even a whole dark section like in "The Sunspire", where there are several pitch black areas full of Tentacles and other critters.
Flashlights provide a focused beam of light that can be turned on and off, i.e. it is good for glancing into dark corners or traversing small dark areas. However, its batteries are limited and after a while it starts to dim and eventually its batteries run out whereupon you have no alternative but to find another Flashlight. Remember Flares, as Flares can provide a more useful general glow, and are more common than Flashlights.
Searchlights are Flashlights on steroids. Searchlights provide a very cold, crisp and bright white illumination over a wider area than the Flashlight which is drab by comparison. Searchlights also have huge power supplies that can keep them going for ages. Collecting a Searchlight half way through Return to Na Pali, it still hasn't run out by the end even with extensive use. Once you have a Searchlight, Flashlights are obsolete.
Jump Boots when activated allow you to make supercharged jumps to reach out of the way locales. Unfortunately, the pitiful power cells within in single player can only supply three turbo-charged jumps, after which the boots are useless and are discarded. Even if you don't activate them, they run out soon anyway, so basically you're scuppered. If you want to use them, you have to use them quickly and accurately.
Super Jump Boots
Actually a tweaked version of the basic Jump Boots actor but we won't go into technicalities now... Super Jump Boots are basically Jump Boots with a much larger power supply. You find them only at "The Source" just before the Queen fight, and they allow you to jump about the balconies to get ammo, health and armour etc. Otherwise, they perform in exactly the same way as regular Jump Boots.
SCUBA Gear gives you about two minutes of breathing underwater, allowing you to venture into deep underwater tunnels or simply to swim normally but not have to surface. After the air runs out of course you have to surface or you'll start to drown, but in the mean time it is a very useful tool for sub-aqua exploration. You don't need to activate it, it comes on automatically. Note that in Return to Na Pali you automatically carry throughout a variant called "Marine SCUBA Gear" which can recharge itself, but as this is not found lying about as a normal item I shan't feature it here as an item in its own right.
Nali Fruit Seeds
You accumulate Nali Fruit Seeds like you do Flares. When you're critically short on health and there's no health around, then you can toss a seed and a Nali Healing Fruit will grow. It takes a short while, but it's not that long to wait. Just make sure that you do wait, because otherwise you won't get the full health boost. Nali Fruit Seeds are quite rare so don't squander them. Use them when they're really needed.
The Acoustic Dampener is actually only used in multiplayer games, but is included in this list for completeness. What it does, when activated, is to silence the sound of your weapon and make it harder for the victim to track your position. Good in combination with the Invisibility I should imagine... only lasts for about 10 seconds of activation.
The Invisibility... well... renders you invisible. It can be turned on and off, giving you overall about 20 seconds of invisible wandering before you are fully visible. However, you do glimpse in to view when you attack, and Skaarj once they have spotted you doing this seem to be able to track you. Most satisfying tactic, sneak in with the Invisibility on, deal a powerful shot to the head with a Rifle or a full Flak bomb, whereupon you will be seen, so deactivate the Invisibility to avoid wasteage. Then, dart out of the room and hide behind a corner. Re-activate and return to the Skaarj. Whack... etc. etc.
The amp is a powerful battery that you can connect to the ASMD or Dispersion Pistol and switch on to significantly increase the power of that weapon. The most powerful Energy Amplifier attacks are Power 5 Dispersion Pistol primary fire, ASMD secondary fire and of course ASMD combo. Note that with the Dispersion Pistol, the amp's effect depends on the version of Unreal you are using. It is much less powerful in 224 than in the retail version; in the reatil version you could kill a Titan in one shot. Either way, it's a rare, powerful item and you shouldn't waste it. Use it when you really need a boost.
Activate the Force Field and a purple shimmering blob appears in front of you that blocks attacks from that quarter. You can use it to block doorways whilst you escape, or simply to confuse an enemy. Once the Force Field is deployed it can't be deactivated; it lasts for about 15 seconds before collapsing.
Drop the Voice Box and it plays a distracting sound of a heated fight before blowing up of its own accord. Theoretically able to distract your enemies but it seems to be ineffective and pointless. Skaarj take no notice, human players can tell. Oh well, it's quite fun to watch the little loudspeakers vibrating I guess...