Disclaimer: Some of these are prior to Plasma Storm.
Professor Juniper/Sycamore - Throw away what you don't need for a fresh hand of 7 cards.
N - Shuffle your hand into your deck (both players) and draw cards equal to the amount of prizes you still have.
Shauna - Kind of the "new N"... shuffle your hand into your deck and draw 5 cards.
Red Card - It's useful if you want to throw off your opponent after they draw some good cards during their last turn. They have to shuffle their hand into their deck, and then draw only 4 cards.
Colress - You have the potential of shuffling your hand into your deck and drawing 10 cards with this (it's based on how many benched Pokemon are in play).
Skyla - Search your deck for any Trainer card.
Bianca - Add cards to your hand, up to 6.
Ultra Ball - Search your deck for any Pokemon, after discarding 2 cards.
Heavy Ball - Search your deck for a Pokemon with retreat cost of 3 or more.
Level Ball - Good for if you play evolution chains in your deck; search for any Pokemon with 90HP or less
Team Plasma Ball - Search your deck for any TP Pokemon. Obviously good to throw into your deck if using a lot of TP Pokemon.
Frozen City (Stadium) - If either player puts an energy on a Pokemon that isn't a TP Pokemon, they get 2 damage counters put on them.
Switch - Get your paralyzed or heavy retreat cost Pokemon out of trouble.
Float Stone - Attach it to a Pokemon and retreat for free (until it's scrapped).
Tool Scrapper - Which brings me to my next card... discard up to 2 tool cards attached your opponent's Pokemon or your own Pokemon.
Hypnotoxic Laser - Poison the opponent with a chance to put it to sleep.
Virbank City Gym (Stadium) - Good if you're using HypnoLaser. Poison does 30 damage instead of 10 between turns.
Professor's Letter - Grab 2 basic energy cards from your deck.
Energy Retrieval - Retrieve 2 basic energies from your discard pile.
Enhanced Hammer - Discard a special energy card attached to one of your opponent's Pokemon.
Computer Search (Only one Ace Spec is allowed per deck) - Throw away 2 cards to search your deck for ANY card.
Dowsing Machine (Only one Ace Spec is allowed per deck) - Toss 2 cards to grab a used Trainer card from your discard pile.
Aaaand that's a good start. There are obviously some others, but some Trainers only really work depending on what strategy you're using. Hope that helps.