For me, Excel is like the software version of a Swiss armyknife. I do all kinds of stuff with it and think it's really useful. Combined with my iPaq 2210 it makes a killer combination that I use daily.
However, I feel that they've stripped Excel a little too much, some functionality and many functions I frequently use are missing (for the moment I would have liked the DAYOFWEEK function but there are more functions I miss).
Before doing my own versions of the most missed functions I have to ask; is there any add-on or perhaps a good source of information regarding writing your own formulas, something that would help me doing or even spare me the work...?