Normally when under the effects of Toxic, your Pokemon takes incrementing amounts of damage: 1/16, 1/8, 3/16, and so on, of their max HP. You can negate this by switching out; when the Pokemon returns, it will only be taking a flat 1/8 damage like regular Poisoning.
Boost Leech Seed's Absorption
Leech Seed is a move well-known for absorbing 1/16 of the target's HP and healing the user of the move by the same amount. However, you can actually boost this damage through the unintended effects of Toxic. When you use Toxic to Badly Poison a foe, and then use Leech Seed on them, Leech Seed's damage will also increment: thus, you regain 1/16, then 1/8, then 3/16, then 1/4, and so on, of your HP. Toxic will also be taking effect in this time, dealing the same amount of damage. In other words, you not only regain tons of HP, but also double the damage of Toxic, from one point of view.
Glitch: Old Man
This glitch requires you to have the Badges allowing the use of Fly and Surf, and Pokemon that can use these moves. You must also be able to reach Fuschia City or Cinnabar Island. First, head to Viridian City and speak with the old man near Route 2 who will teach you to catch Pokemon; view his tutorial. After doing so, Fly to Fuschia or Cinnabar and Surf to the Seafoam Islands. Once there, Surf on the coastline of the islands. As you Surf, you may encounter a number of odd Pokemon. These Pokemon will differ, depending on the name you used as that of your Trainer. For example, the name "DANIEL" can make Snorlax or Electrode appear, among others. However, a common characteristic of them is that they are often above Level 100 and thus very powerful: if you manage to catch one somehow, you essentially have a very powerful Pokemon, and can even level it up further, as high as Level 255. (Note that some Pokemon may evolve into glitch Pokemon because of this.) That is the first benefit of this simple glitch. Most people, regardless of name, should be able to encounter one of two Pokemon: Missingno. or 'M, both of which are glitch Pokemon. These two have special effects on the item in the sixth slot of your item bag, but that effect depends on whether you catch or defeat the Pokemon. If you defeat the Pokemon (or otherwise just do not catch it), the item in your Bag's sixth slot will be changed so that you have 128 of it; if you catch it, you will have 255 of it. (This is assuming you use an item that you can have multiple ones of.) This is very useful for mass-cloning valuable items, such as Master Balls, Rare Candies, or PP Maxes, or to clone TMs to sell for obscene amounts of money.
Hyper Beam Recharge Perpetuation
This glitch can be executed on Pokemon who have used Hyper Beam and are Frozen during their recharging turn. If you use Haze on this Pokemon, they will thaw out - however, they will always be recharging from their Hyper Beam, meaning they and their Trainer can do nothing until you defeat it. You can capitalize on this with healing across the party, massive stat boosts, and inevitably killing the Pokemon.
Invulnerability
This glitch is executed when a Pokemon using Fly or Dig becomes Paralyzed before the turn on which they attack: it is only done by being Paralyzed when you are high up in the air when using Fly. If such occurs, you will be invulnerable. This glitch lasts until your Pokemon uses Fly or Dig again and is successful in doing so, you switch Pokemon, the Pokemon faints, or the battle ends.
Struggle
Normally in Pokemon, the move Struggle (used when a Pokemon is at 0 PP for all moves) is intended to be a typeless move. However, in these games it is mistakenly coded as a Normal-type move. This means that you can use a Ghost-type Pokemon against them and easily win as they won't be able to hurt you.
Alter Toxic Damage...