I use BerryDailer, and it's amazing for using calling cards. I used to have to either memorize or write down numbers that I'm not familiar with before calling my calling card, but now it only takes two clicks. I've set up 4 calling cards so far and it's been working great.
RE: itokk On the other thread someone mentioned a rate of 5 cents instead of calling card's 1 cent. If you do the math, $19.99/$0.04 = 500 minutes, i.e. if you talk more than 500 minutes, the savings you get from your calling card would cover the cost of the program. Everything you use afterwards would be a net saving. I know for sure that I'll go over 500 minutes in a month...
On the other hand, if you are 1) over your outgoing call bucket AND 2) have unlimited incoming, then you may consider using itokk as it'll be cheaper than paying just for the local outgoing call to use the calling card.