I used to use the online Battle Calc.. http://www.aeproject.org/calculators/battle
I like the copy paste feature, so i dont have to enter all the tech data, I just need to use a scout to get tech levels, then copy and paste the fleet data.
It has the option to "waves" so you can work out a multi drop.
Not perfect, but seems to be fairly accurate.
Also have used ACE Battlecalc in the past, but the lack of "Cut and Paste" is a drawback, but is accurate when set up properly.
Have just got hold of KaHaR Battlecalc now, so will test that out on my next few hits to see how it compares to other hits. Though to be honest, I can do the hits without, but usually only use it for multiple fleet hits to make sure I have enough attack force to complete all hits and get away with all the derbs..