  We present a method of general applicability for finding exact or accurate
approximations to bond percolation thresholds for a wide class of lattices. To
every lattice we sytematically associate a polynomial, the root of which in
$[0,1]$ is the conjectured critical point. The method makes the correct
prediction for every exactly solved problem, and comparison with numerical
results shows that it is very close, but not exact, for many others. We focus
primarily on the Archimedean lattices, in which all vertices are equivalent,
but this restriction is not crucial. Some results we find are kagome:
$p_c=0.524430...$, $(3,12^2): p_c=0.740423...$, $(3^3,4^2): p_c=0.419615...$,
$(3,4,6,4):p_c=0.524821...$, $(4,8^2):p_c=0.676835...$, $(3^2,4,3,4)$:
$p_c=0.414120...$ . The results are generally within $10^{-5}$ of numerical
estimates. For the inhomogeneous checkerboard and bowtie lattices, errors in
the formulas (if they are not exact) are less than $10^{-6}$.
