I really ought to do a post on estimation and developing mathematical intuition one of these days

Till then, check out this decidedly cool offering from MIT's reliably cool Open Courseware (via DeLong):

18.098 / 6.099 Street-Fighting Mathematics

This course teaches the art of guessing results and solving problems without doing a proof or an exact calculation. Techniques include extreme-cases reasoning, dimensional analysis, successive approximation, discretization, generalization, and pictorial analysis. Applications include mental calculation, solid geometry, musical intervals, logarithms, integration, infinite series, solitaire, and differential equations. (No epsilons or deltas are harmed by taking this course.) This course is offered during the Independent Activities Period (IAP), which is a special 4-week term at MIT that runs from the first week of January until the end of the month.