I'm playing a Psion (Shaper) right now. Currently 5th level. Here's a short list of the powers I've found most versatile, and which I've had occasion to play with:
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Minor Creation (Shaper 1): I haven't been allowed to make poison or anything like that, and this power still has had huge applications. So much so that the DM made it part of the plot. The party had to pull off a
Raiders of the Lost Ark-style maneuver and my character was the only one who could do it. Other than that, I've made ladders, bridges, caltrops, acid, rope...lots of stuff.
Astral Construct (Shaper 1): Hoo, boy, have these things been a blessing! For all the reasons others have mentioned, this is a great power. Also, you can use it to set up flanking opportunities and rescue party members from a jam. Like popping one with a swim speed to retrieve a hide armor-clad Druid with no ranks in Swim who decides to try to cross a fast-moving river...and who forgets that he could just turn into a fish.
Matter Agitation (Psion 1): This should be every low-level Psion's preferred method of attack. It's guaranteed damage with no save and no attack roll, and preserves your precious power points. For levels 1-3, it's often better to use this than to manifest a construct. Plus it has some utility: starting fires, like the OP mentioned. Not a power to hang onto for your whole career, but one that will see you through the first several levels.
Control Sound (Psion 2): This power is tons of fun. I've used it to, let's see...bypass several encounters by imitating the sound of a dangerous animal, impress a noblewoman with a jaunty tune, set off traps (alchemical vials set up as "mines"), ensure that my character made an impressive entrance, and to Rickroll my fellow party members.
Levitate (Psion 2): Another power that's good for low levels, until you get a power that allows you to fly. A great way to avoid damage. Synergizes well with both Astral Construct and Minor Creation. Levitate and then use an AC to tow you around. Explore huge pit in the earth by creating a rope bridge then Levitating down to take a look. Also works well with other party members' class abilities. Party being tracked? Levitate and be the Druid's Psion Balloon until you've thrown 'em off the trail.
Time Hop (Psion 3): Others have mentioned how versatile this power is. Got a party member who's bleeding to death? Time Hop 'em out until the healer can stabilize 'em. Big brute with a two-handed weapon and Power Attack bearing down on you? Target his weak Will save with this power and then prepare a few ACs to knock him silly when he returns. Need to create a diversion in a camp full of slavers so you can free the slaves? Hop out the central pole of their pavilion tent and watch 'em scramble. Hiding spot of you or a party member about to be discovered by some nasty thing? Send yourself or your companion out of the time stream until that thing moves on. So many, many uses.
Energy Wall (Psion 3): No, its damage potential isn't that great, but there's still a lot you can do with it, not the least of which is turn ANYTHING, even a solid block of adamantine, into dust. With the Sonic version you get your own personal wrecking ball/jackhammer/atom smasher. Since it lasts Concentration+1 round/level, all you need is time and even the strongest object will be reduced to rubble, and the rubble to tinier rubble, and so on and so forth. Picking it will also cover the AoE you need to take out swarms. Furthermore, its duration allows you to create rings within rings within rings, and if you can keep a bad guy pinned down inside those rings it all but ensures their death.
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Here are a few other powers which I can imagine would be quite versatile, but which I haven't played with yet:
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Fabricate (Shaper 4) and Greater Fabricate (Shaper 6): Make just about anything. With the latter, make even MORE!
Divination (Psion 4): Unless you roll badly, it could be the only information-gathering power you ever need.
Wall of Ectoplasm (Psion 4): Not a very strong wall, but good in a pinch. Depending on what your DM allows, you could potentially make a boat out of a hemisphere-shaped wall, or a Giant Hamster Ball of Doom with a full sphere.
Metamorphosis (Egoist 4): Very good on its own. Ridiculously powerful when you also take Metamorphic Transfer.
Disintegrate (Psion 6): Highly damaging when used as an attack and incredibly versatile when applied to your surroundings.
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Two metapsionic feats that allow you to do things you couldn't otherwise do:
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Burrowing Power: First, it turns Ectoplasmic Cocoon into a true save-or-die. Cocoon an enemy and then drop a Burrowing Energy Wall on 'em. Furthermore, it can let you see through walls with certain powers, attack enemies who are behind a closed door, and generally get at things ordinarily out of reach.
Transdimensional Power: Negates a whole lot of strong defensive abilities, like etherealness and incorporeality. Makes it so you don't need a Force effect power on your list. Supercharges Touchsight.
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Finally, here are some links to other threads that have discussions of the usefulness of many powers:
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3.5 Psion Buildguide and Compendium (carnivore's iconic guide; most of the below links were first found there, before Wizards broke 'em)
The 3.5 Psion Power Selection GuideMust-have Psionics?The Minimalist Psion Offense ChallengeGive Me Your Most Useful Power/Feat Combinations!Power listPsion//Warblade gestalt - which powers?What Is the Most Powerful Psionic Power? (a good resource, overall, though the author has an obvious bias towards Kineticist discipline-specific powers)
Ardent handbook (also useful for Psions)
PhaedrusXY's build compilation[/spoiler]
