Terraria – pour water out of a bottle

The liquid physics in Terraria are not completely precise. You can abuse it to duplicate them. Good thing, too, since transporting a decent amount of lava/water anywhere is a huge pain.

1. Infinite container: If you make a liquid container large enough, the amount that drains each frame is rounded down to nothing, for some reason. For example, a 49×25 liquid container with a 1 block hole will create an infinite amount of liquid (video example of this). This will break if liquids are settled (via the server console or when saving and loading the map).
2. Liquid splitting: When a liquid falls onto a single block and could flow down both the left and right edge, it will. Somehow, this leaves you with more liquid in total than you started with.
3. Bucket weirdness: Buckets create 1 block of liquid when they're emptied, but filling them seems to require less. If you place a block, let some of it flow and then quickly pick it up again, you will have a full bucket, but some of the liquid will remain.

Applying #2 and #3, this is a very simple liquid duping setup:

The lava is placed on the edge, where it flows into both the container and the 1-block "basin" on the right side. Both "portions" are enough to be picked up by a bucket again (though in this case, the first scoop will be spread too thinly in the container).

From 1.1 With Love

Spawn Switching

By using a on/off wire trigger block you can effectively "open" your bed's wall and invalidate the room it's in, leading you to spawn back at the middle of your world. If you're close enough (wiring only works for 2000 blocks or so) you can just hook up a switch at your spawn that'll change the blocks again and allow you to spawn back at your home base. Otherwise, you might be able to coax a contraption with a statue spawner/pressure plate/lava block to toggle it for you automatically.

This post on the TerrariaOnline forums in particular has an interesting bird-based reset circuit that seems to be fairly popular, but you'll need a bird spawner.

Digging the hard way

From your main base, just dig a hole straight down (the classic "Hellevator", if you will). When you get tired of exploring your caves, just start digging left/right towards your shaft. It'll take a while, but you'll at least probably find some goodies (more caves, ores...) on the way, and getting back will be a lot easier. You can even use a depth meter to align all your "escape routes" to the same depth, so that eventually you can just have one long path across the world that speeds up travel.

Previously, on Terraria...

A trick that worked in 1.0.5 was to start up a multiplayer server and explore with a friend (or just explore with two characters locally), then follow the instructions:

1. Build a room with a bed. A room isn't necessarily required, but will keep the other person safe. It's best to put the bed in a corner.
2. Set your spawn point to the bed.
3. Break the bed and use a mirror. Logging out might work but I don't make any promises.
4. Do whatever you need at your spawn point. When you're done, let your friend know.
5. Put the bed back down in the exact same spot. This is important.
6. Use the mirror again.

If you did it properly, you should travel back to where the bed is. This might not work in 1.0.6, though, and it's only good for exploring together -- if either of you get separated, you'll have to travel back manually.

If you're exploring solo, your only real option is just to take the time to improve the path. Wooden platforms over minor gaps and flattening out bumps will help a lot with horizontal movement, and creating a double shaft with a soft landing area (e.g. water) on one side and a wooden platform tower on the other will make vertical travel easier. Using background walls other than dirt will also stop the enemies from spawning.