Iterative solvers for linear problems. The following solvers are available:
Solver SPD Transpose Notes
BiCG Yes
BiCGstab
CG Yes
CGS
Chebyshev Yes Needs extremal eigenvalues
GMRES Restarted version
IR
QMR Yes Uses left and right preconditioning

SPD means that the matrix must be symmetrical, positive definite, while transpose means that transpose matrix/vector multiplication and preconditioning is necessary.