makehumancommunity.org

The default rig from makehuman is pretty nice. When loading it with mhx2 it's even possible to turn it into a IK rig.
In this case, the lower legs sometimes transform in a stupid way, because they consist of two parts, even if they normally should be solid. Why?
Also it's not as advanced as the other supported IK rigs from mhx2 like MHX or rigify.
But these rigs don't contain toes, and the toungue also is not as flexible as with the default rig.
So what's the best way to get an IK rig, that's easy to use and flexible?
Probably some modifications to the default makehuman rig.

As in any other engineering effort, you'll need to ask the question: "What actions is my model/rig going to do?"

Most of the IK rig tutorials I've looked at are focused on cartoony walk cycles; I don't use them, because I've done my own, that have more a reactive look (the leg twists out at the hip on striking the ground, there is a recoil through the whole body, etc). What I've found IK the most useful for is unconventional, fluid arm and leg movements. To that extent, I altered a walk cycle tutorial to give this rig:

http://www.geekopolis.ca/valentina/ellieE2_276b.blend

Which looks like this:

http://www.makehumancommunity.org/forum/viewtopic.php?f=14&t=14558

It works great for kung fu moves, especially.

There is a snap on IK rig in the MH-Community Blender plugin. It really only works on the game rig, because of the concerns you mention. The export process tested was export GAME_RIG as an MHX2 file. Import without overrides. In addition to the IK control bones being added, some location locks are turned off. This helps when hands go over the head.