There are several universities that offer courses through distance learning programs.
University of Wisconsin-Madison offers several lower and upper division mathematics courses, including Calc II, Calc III, ODE, Modern Algebra, and Complex Analysis.
http://www.dcs.wisc.edu/lsa/indlearn/math.htm
University of Minnesota offers Calc II, Calc III, and Linear Differential Equations. http://idlwebdb.cce.umn.edu/searchresults.asp?textsubj=math&Subjects=Search
University of Illinois at Urbana-Champaign has several upper division math courses aimed specifically at engineers.
http://netmath.uiuc.edu/emo/index.php
It might be helpful to narrow the scope of your inquiry. Are you interested in just learning more, or are you interested in something for a specific purpose?
If, as you suggest, you're interested in just adding tools to the toolbox, I would think steering toward Linear Algebra, Numerical Analysis, Number Theory and Statistics courses would give you the most useful and applicable knowledge. If you did well at Calc I, you can probably get by without Calc II (though it may be a pre-requisite for some Numerical Analysis courses). I mention this because Calc II is considered by many (including me) to be the most difficult lower division math course mainly because of the tedium of the problems. I still have Integration by Trigonometric Substitution scars.