http://veekun.com/dex/gadgets/stat_calculator is my current favorite IV calc. It has nice colored bars to show you what your range should be, and that gives it a nice friendly read-out, unlike Serebii's calculator. It's also one of the easier to plug in. Psypokes' is good, but Veekun's is just much cleaner to look at.
As for simplifying your calculations, I do what Jert does, but in reverse. I separate my pokemon into bad, alright, and optimum in terms of Nature. For example, Modest and Adamant are optimum, while Lonely and Timid are alright. I check the IVs of all the optimum-Natured pokemon. If the average of all the IVs (add the min+max, divide by 6, divide by 2) beats 20, then we've got a great pokemon on our hands. If none beat 20, then we go to the alright-Natured and find the best one there in terms of IVs. Out of 27 Digletts I only needed to compare 3 because only 3 had good Natures, so it's safe to assume that you won't get bogged down too much by stats.
Oh, and for those that wonder about Nature vs. IV, let's look at that Blissey's stats.
(135*2)+0[iV]+63[EV]+5=338
A good Nature makes this 372, while perfect IVs makes it 369. However, IVs spread across all stats, not just 2 like a Nature does.
I really liked this guide; it's sure to help a lot of players out.