The following resources survey some popular numerical methods for inverse kinematics problems:
Samuel R. Buss. Introduction to Inverse Kinematics with Jacobian Transpose, Pseudoinverse and Damped Least Squares methods
Bill Baxter. Fast Numerical Methods for Inverse Kinematics
Chris Welman. Inverse Kinematics and Geometric Constraints for Articulated Figure Manipulation
Buss's survey may be particularly interesting, because it explicitly discusses multiple limbs.
IK systems for animation must generally support multiple, possibly conflicting, constraints. For example, one arm can hold on to a rail while the other arm reaches for a target.
- Jianmin Zhao and Norman Badler. Inverse Kinematics Positioning Using Nonlinear Programming for Highly Articulated Figures
6 dof industrial robots generally have closed form IK solutions, as mentioned by Andrew and explained in e.g. Craig: Introduction to Robotics. More useful for figure animation are methods for 7 dof human-like arms and legs: