To be able to enter test commands, you will have to start a Practice Game (Create Lobby), then go to Game Setup and "Enable cheats":



Now start the game. While in the game, you can enter commands by typing them into the chat-window.

Following is a list of available commands.

-lvlup <amount>
For example: -lvlup 10
Increases your heroes' level by <amount>. Max level you can reach with this is level 25. No negative values allowed (i.e. you can't "level down").
Console command: dota_dev hero_level <amount>

-respawn
Respawns your hero at the fontain (no matter if he was dead or not).
Console command: dota_dev hero_respawn

-refresh
Refreshes (resets) Mana, Cooldowns, HP, ... of your hero(es).
Console command: dota_dev hero_refresh

-gold <amount>
For example: -gold 1000
Increases your gold by <amount>. Positive and negative values are allowed. Max gold is 16383.
Console command: dota_dev player_givegold <amount>

-item <item>
For example: -item item_blade_mail
Gives your hero <item>. You find the item names in scripts/npc/items.txt
I made a list here: http://typhox.net/misc/Dota_items.txt

-startgame
Skips the timer to the first creepspawn.
Console command: dota_dev forcegamestart

-killwards
Destroys all wards (to be confirmed)

-createhero <unitname>
Examples:
-createhero Mirana
-createhero npc_dota_creep_goodguys_melee
Creates a hero/unit/building <unitname> under your control at your mouse cursor position. Heroes will be taken control by a bot, so you can't buy items for them on the regular way. However, that bot is idle, i.e. he does not do anything, but buys items in the beginning.
You can't create heroes that aren't in the game yet (only in the developer version), such as Bane or Silencer.
You find the names in scripts/npc/npc_units.txt and scripts/npc/npc_heroes.txt.
There are many names that are working. For example the entry under "SoundSet" is sometimes also valid, so you can create Mirana on (at least) three ways:
-createhero Mirana
-createhero Hero_Mirana
-createhero npc_dota_hero_mirana
I'm currently looking for a list

-levelbots <amount>
Levels all bot heroes on your side by <amount>.
See: -lvlup

-givebots <item>
gives all bots item <item>.
For more information, see -item

-wtf, -unwtf
Seems to be currently only working in the developer/debug version sadly...

-timescale
Seems to be not working yet.

-spawnneutrals
(re)spawns neutral creeps.

-spawncreeps
Spawns a wave of creeps (Warning: If you do that too often so that there are too many creeps at once in the game, the server will disconnect you.)

-disablecreepspawn
Disables creepspawn.

-enablecreepspawn
Enables creepspawn.

-killcreeps
Seems to be not working.

-allvision
Gives vision over Radiant and Dire.

-normalvision
Reverts vision back to normal from -allvision command.

-clearwards
Removes Observerwards from the map (not sentry wards?).