The Sewers of Dross
The Forlorn Hollow
The 7 Planes of Hehenna
Warp Zones

If I get enough clicks on the google ads, I'll be much more likely to complete this website.

The latest version of Gish (download the patch if you need to) includes a manual to help acquaint you with the basics.  I recommend reading that, in addition to this page.

Navigating obstacles

Spikes

In addition to poking Gish (causing damage on touch), spikes have a tendency to impale him and stick, making it difficult to escape a spike pit once you've fallen into one.  To make matters worse, the harder you push on the spikes the more damage they do, so it's not easy to build up a good jump to escape.

To counter the ill effects of the spikes, try staying in slick mode while you're touching them (obviously, don't use sticky at the same time) to keep the spikes from sticking to you.  Holding down the jump key can also help to reduce the pressure with which you squish into them.

Falling

If you find yourself falling, it's usually a good idea to press and hold the Jump button so that you don't die when you hit the ground.  If you'd rather hit the ground con gusto, try holding the Heavy button instead... just make sure you don't let go until after you hit the ground.

Breaking things

If you apply enough pressure to certain blocks, they will break apart, giving you 10 points and often revealing secret areas.  To do this, it's often easiest to use HEAVY mode while you fall or run into them.  Sometimes this isn't enough, though.  To give it that last little bit of umph, try releasing the HEAVY button just before you hit them and then pressing it again once Gish is squished against them.

Killing enemies

One of my biggest gripes about this game is the lack of different types of enemies.  You've got the little ones that jump (just smash them without letting their teeth touch you), and the big ones that punch.  Avoid the bottom half of their bodies while hitting their heads as hard as possible.  Usually knocking their heads off center works better than hitting them straight down, and you can use the same strategies described above for "Breaking Things" to apply enough pressure to kill them.

Bug exploits & Cheats

End Task

Jon Richardson says that if you push Ctrl-Alt-Delete while playing most levels, the game will allow you (the next time you start the game) to start on the same level without acting like you've died.  A notable exception is the Isle of the Dead level, where doing this causes the game to assume that you've beat the level.  It's an easy(ish) way to beat ludicrous mode.  Beating ludicrous mode gives you Small Gish mode.

Practice any level

Though I haven't tried this, I've heard you can replace the "playroom.lvl" file in the Gish\level directory with any other level in that directory (obviously you'll want to back up the original first).  This means that going into the Playroom on the game will take you to that level.  In addition to letting you practice any level as much as you want, this also lets you play the "Super Secret Levels" without actually getting to them.  Also, if you have the latest patch and beat the game in normal mode, you should be able to jump to any normal level.